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L_0772 | inside the atom | T_3971 | Sometimes atoms lose or gain electrons. Then they become ions. Ions have a positive or negative charge. Thats because they do not have the same number of electrons as protons. If atoms lose electrons, they become positive ions, or cations. If atoms gain electrons, they become negative ions, or anions. Consider the example of fluorine in Figure 5.5. A fluorine atom has nine protons and nine electrons, so it is electrically neutral. If a fluorine atom gains an electron, it becomes a fluoride ion with a negative charge of minus one. | text | null |
L_0772 | inside the atom | T_3972 | Some atoms of the same element may have different numbers of neutrons. For example, some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in number of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same chemical properties. Thats because they have the same numbers of protons and electrons. For a video explanation of isotopes, go to this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0772 | inside the atom | T_3973 | Hydrogen is a good example of isotopes because it has the simplest atoms. Three isotopes of hydrogen are modeled in Figure 5.6. Most hydrogen atoms have just one proton and one electron and lack a neutron. They are just called hydrogen. Some hydrogen atoms have one neutron. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium. | text | null |
L_0772 | inside the atom | T_3974 | For most other elements, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13. Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon? You can learn more about this isotope at the URL below. It is used by scientists to estimate the ages of rocks and fossils. | text | null |
L_0772 | inside the atom | T_3975 | Remember the quarks from the first page of this chapter? Quarks are even tinier particles of matter that make up protons and neutrons. There are three quarks in each proton and three quarks in each neutron. The charges of quarks are balanced exactly right to give a positive charge to a proton and a neutral charge to a neutron. It might seem strange that quarks are never found alone but only as components of other particles. This is because the quarks are held together by very strange particles called gluons. | text | null |
L_0772 | inside the atom | T_3976 | Gluons make quarks attract each other more strongly the farther apart the quarks get. To understand how gluons work, imagine holding a rubber band between your fingers. If you try to move your hands apart, they will be pulled back together by the rubber band. The farther apart you move your hands, the stronger the force of the rubber band pulling your hands together. Gluons work the same way on quarks inside protons and neutrons (and other, really rare particles too). If you were to move your hands apart with enough force, the rubber band holding them together would break. The same is true of quarks. If they are given enough energy, they pull apart with enough force to "break" the binding from the gluons. However, all the energy that is put into a particle to make this possible is then used to create a new set of quarks and gluons. And so a new proton or neutron appears. | text | null |
L_0772 | inside the atom | T_3977 | The existence of quarks was first proposed in the 1960s. Since then, scientists have done experiments to show that quarks really do exist. In fact, they have identified six different types of quarks. However, much remains to be learned about these tiny, fundamental particles of matter. They are very difficult and expensive to study. If you want to learn more about them, including how they are studied, the URL below is a good place to start. | text | null |
L_0772 | inside the atom | T_3978 | QUEST journeys back to find out how physicists on the UC Berkeley campus in the 1930s, and at the Stanford Linear Accelerator Center in the 1970s, created "atom smashers" that led to key discoveries about the tiny constituents of the atom and paved the way for the Large Hadron Collider in Switzerland. For more information on particle accelerators, see http://science.kqed.org/quest/video/homegrown-particle-accelerators/ . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0773 | history of the atom | T_3979 | The history of the atom begins around 450 B.C. with a Greek philosopher named Democritus (see Figure 5.7). Democritus wondered what would happen if you cut a piece of matter, such as an apple, into smaller and smaller pieces. He thought that a point would be reached where matter could not be cut into still smaller pieces. He called these "uncuttable" pieces atomos. This is where the modern term atom comes from. Democritus was an important philosopher. However, he was less influential than the Greek philosopher Aristotle, who lived about 100 years after Democritus. Aristotle rejected Democrituss idea of atoms. In fact, Aristotle thought | text | null |
L_0773 | history of the atom | T_3980 | Around 1800, a British chemist named John Dalton revived Democrituss early ideas about the atom. Dalton is pictured in Figure 5.8. He made a living by teaching and just did research in his spare time. Nonetheless, from his research results, he developed one of the most important theories in science. | text | null |
L_0773 | history of the atom | T_3981 | Dalton did many experiments that provided evidence for atoms. For example, he studied the pressure of gases. He concluded that gases must consist of tiny particles in constant motion. Dalton also researched the properties of compounds. He showed that a compound always consists of the same elements in the same ratio. On the other hand, different compounds always consist of different elements or ratios. This can happen, Dalton reasoned, only if elements are made of tiny particles that can combine in an endless variety of ways. From his research, Dalton developed a theory of the atom. You can learn more about Dalton and his research by watching the video at this URL: (9:03). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0773 | history of the atom | T_3982 | The atomic theory Dalton developed consists of three ideas: All substances are made of atoms. Atoms are the smallest particles of matter. They cannot be divided into smaller particles. They also cannot be created or destroyed. All atoms of the same element are alike and have the same mass. Atoms of different elements are different and have different masses. Atoms join together to form compounds. A given compound always consists of the same kinds of atoms in the same ratio. Daltons theory was soon widely accepted. Most of it is still accepted today. The only part that is no longer accepted is his idea that atoms are the smallest particles. Scientists now know that atoms consist of even smaller particles. | text | null |
L_0773 | history of the atom | T_3983 | Dalton incorrectly thought that atoms are tiny solid particles of matter. He used solid wooden balls to model them. The sketch in the Figure 5.9 shows how Daltons model atoms looked. He made holes in the balls so they could be joined together with hooks. In this way, the balls could be used to model compounds. When later scientists discovered subatomic particles (particles smaller than the atom itself), they realized that Daltons models were too simple. They didnt show that atoms consist of even smaller particles. Models including these smaller particles were later developed. | text | null |
L_0773 | history of the atom | T_3984 | The next major advance in the history of the atom was the discovery of electrons. These were the first subatomic particles to be identified. They were discovered in 1897 by a British physicist named J. J. Thomson. You can learn more about Thomson and his discovery at this online exhibit: . | text | null |
L_0773 | history of the atom | T_3985 | Thomson was interested in electricity. He did experiments in which he passed an electric current through a vacuum tube. The experiments are described in Figure 5.10. Thomsons experiments showed that an electric current consists of flowing, negatively charged particles. Why was this discovery important? Many scientists of Thomsons time thought that electric current consists of rays, like rays of light, and that it is positive rather than negative. Thomsons experiments also showed that the negative particles are all alike and smaller than atoms. Thomson concluded that the negative particles couldnt be fundamental units of matter because they are all alike. Instead, they must be parts of atoms. The negative particles were later named electrons. | text | null |
L_0773 | history of the atom | T_3986 | Thomson knew that atoms are neutral in electric charge. So how could atoms contain negative particles? Thomson thought that the rest of the atom must be positive to cancel out the negative charge. He said that an atom is like a plum pudding, which has plums scattered through it. Thats why Thomsons model of the atom is called the plum pudding model. You can see it in Figure 5.11. It shows the atom as a sphere of positive charge (the pudding) with negative electrons (the plums) scattered through it. | text | null |
L_0773 | history of the atom | T_3987 | A physicist from New Zealand named Ernest Rutherford made the next major discovery about atoms. He discovered the nucleus. You can watch a video about Rutherford and his discovery at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0773 | history of the atom | T_3988 | In 1899, Rutherford discovered that some elements give off positively charged particles. He named them alpha particles (a). In 1911, he used alpha particles to study atoms. He aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it. If Thomsons plum pudding model were correct, the alpha particles should be deflected a little as they passed through the foil. Why? The positive "pudding" part of gold atoms would slightly repel the positive alpha particles. This would cause the alpha particles to change course. But Rutherford got a surprise. Most of the alpha particles passed straight through the foil as though they were moving through empty space. Even more surprising, a few of the alpha particles bounced back from the foil as though they had struck a wall. This is called back scattering. It happened only in very small areas at the centers of the gold atoms. | text | null |
L_0773 | history of the atom | T_3989 | Based on his results, Rutherford concluded that all the positive charge of an atom is concentrated in a small central area. He called this area the nucleus. Rutherford later discovered that the nucleus contains positively charged particles. He named the positive particles protons. Rutherford also predicted the existence of neutrons in the nucleus. However, he failed to find them. One of his students, a physicist named James Chadwick, went on to discover neutrons in 1932. You learn how at this URL: . | text | null |
L_0773 | history of the atom | T_3990 | Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread out everywhere in an atom. It is all concentrated in the tiny nucleus. The rest of the atom is empty space, except for the electrons moving randomly through it. In Rutherfords model, electrons move around the nucleus in random orbits. He compared them to planets orbiting a star. Thats why Rutherfords model is called the planetary model. You can see it in Figure 5.13. | text | null |
L_0773 | history of the atom | T_3990 | Rutherfords discoveries meant that Thomsons plum pudding model was incorrect. Positive charge is not spread out everywhere in an atom. It is all concentrated in the tiny nucleus. The rest of the atom is empty space, except for the electrons moving randomly through it. In Rutherfords model, electrons move around the nucleus in random orbits. He compared them to planets orbiting a star. Thats why Rutherfords model is called the planetary model. You can see it in Figure 5.13. | text | null |
L_0774 | modern atomic theory | T_3991 | Bohrs research focused on electrons. In 1913, he discovered evidence that the orbits of electrons are located at fixed distances from the nucleus. Remember, Rutherford thought that electrons orbit the nucleus at random. Figure 5.14 shows Bohrs model of the atom. | text | null |
L_0774 | modern atomic theory | T_3992 | Basic to Bohrs model is the idea of energy levels. Energy levels are areas located at fixed distances from the nucleus of the atom. They are the only places where electrons can be found. Energy levels are a little like rungs on a ladder. You can stand on one rung or another but not between the rungs. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model of an atom in Figure 5.15 has six energy levels. The level with the least energy is the one closest to the nucleus. As you go farther from the nucleus, the levels have more and more energy. Electrons can jump from one energy level to another. If an atom absorbs energy, some of its electrons can jump to a higher energy level. If electrons jump to a lower energy level, the atom emits, or gives off, energy. You can see an animation at this happening at the URL below. | text | null |
L_0774 | modern atomic theory | T_3992 | Basic to Bohrs model is the idea of energy levels. Energy levels are areas located at fixed distances from the nucleus of the atom. They are the only places where electrons can be found. Energy levels are a little like rungs on a ladder. You can stand on one rung or another but not between the rungs. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels. The model of an atom in Figure 5.15 has six energy levels. The level with the least energy is the one closest to the nucleus. As you go farther from the nucleus, the levels have more and more energy. Electrons can jump from one energy level to another. If an atom absorbs energy, some of its electrons can jump to a higher energy level. If electrons jump to a lower energy level, the atom emits, or gives off, energy. You can see an animation at this happening at the URL below. | text | null |
L_0774 | modern atomic theory | T_3993 | Bohrs idea of energy levels is still useful today. It helps explain how matter behaves. For example, when chemicals in fireworks explode, their atoms absorb energy. Some of their electrons jump to a higher energy level. When the electrons move back to their original energy level, they give off the energy as light. Different chemicals have different arrangements of electrons, so they give off light of different colors. This explains the blue- and purple- colored fireworks in Figure 5.16. | text | null |
L_0774 | modern atomic theory | T_3994 | In the 1920s, physicists discovered that electrons do not travel in fixed paths. In fact, they found that electrons only have a certain chance of being in any particular place. They could only describe where electrons are with mathematical formulas. Thats because electrons have wave-like properties as well as properties of particles of matter. It is the "wave nature" of electrons that lets them exist only at certain distances from the nucleus. The negative electrons are attracted to the positive nucleus. However, because the electrons behave like waves, they bend around the nucleus instead of falling toward it. Electrons exist only where the wave is stable. These are the orbitals. They do not exist where the wave is not stable. These are the places between orbitals. | text | null |
L_0774 | modern atomic theory | T_3995 | Today, these ideas about electrons are represented by the electron cloud model. The electron cloud is an area around the nucleus where electrons are likely to be. Figure 5.17 shows an electron cloud model for a helium atom. | text | null |
L_0774 | modern atomic theory | T_3996 | Some regions of the electron cloud are denser than others. The denser regions are areas where electrons are most likely to be. These regions are called orbitals. Each orbital has a maximum of just two electrons. Different energy levels in the cloud have different numbers of orbitals. Therefore, different energy levels have different maximum numbers of electrons. Table 5.1 lists the number of orbitals and electrons for the first four energy levels. Energy levels farther from the nucleus have more orbitals. Therefore, these levels can hold more electrons. Energy Level Number of Orbitals 1 2 3 4 1 4 9 16 Max. No. of Electrons (@ 2 per orbital) 2 8 18 32 Figure 5.18 shows the arrangement of electrons in an atom of magnesium as an example. The most stable arrange- ment of electrons occurs when electrons fill the orbitals at the lowest energy levels first before more are added at higher levels. You can learn more about orbitals and their electrons at the URL below: | text | null |
L_0774 | modern atomic theory | T_3996 | Some regions of the electron cloud are denser than others. The denser regions are areas where electrons are most likely to be. These regions are called orbitals. Each orbital has a maximum of just two electrons. Different energy levels in the cloud have different numbers of orbitals. Therefore, different energy levels have different maximum numbers of electrons. Table 5.1 lists the number of orbitals and electrons for the first four energy levels. Energy levels farther from the nucleus have more orbitals. Therefore, these levels can hold more electrons. Energy Level Number of Orbitals 1 2 3 4 1 4 9 16 Max. No. of Electrons (@ 2 per orbital) 2 8 18 32 Figure 5.18 shows the arrangement of electrons in an atom of magnesium as an example. The most stable arrange- ment of electrons occurs when electrons fill the orbitals at the lowest energy levels first before more are added at higher levels. You can learn more about orbitals and their electrons at the URL below: | text | null |
L_0775 | how elements are organized | T_3997 | Mendeleev was a teacher as well as a chemist. He was writing a chemistry textbook and needed a way to organize the elements so it would be easier for students to learn about them. He made a set of cards of the elements, similar to a deck of playing cards, with one element per card. On the card, he wrote the elements name, atomic mass, and known properties. He arranged and rearranged the cards in many different ways, looking for a pattern. He finally found it when he placed the elements in order by atomic mass. | text | null |
L_0775 | how elements are organized | T_3998 | You can see how Mendeleev organized the elements in Figure 6.2. From left to right across each row, elements are arranged by increasing atomic mass. Mendeleev discovered that if he placed eight elements in each row and then continued on to the next row, the columns of the table would contain elements with similar properties. He called the columns groups. They are sometimes called families, because elements within a group are similar but not identical to one another, like people in a family. Mendeleevs table of the elements is called a periodic table because of its repeating pattern. Anything that keeps repeating is referred to as periodic. Other examples of things that are periodic include the monthly phases of the moon and the daily cycle of night and day. The term period refers to the interval between repetitions. In a periodic table, the periods are the rows of the table. In Mendeleevs table, each period contains eight elements, and then the pattern repeats in the next row. | text | null |
L_0775 | how elements are organized | T_3999 | Did you notice the blanks in Mendeleevs table (Figure 6.2)? They are spaces that Mendeleev left for elements that had not yet been discovered when he created his table. He predicted that these missing elements would eventually be discovered. Based on their position in the table, he could even predict their properties. For example, he predicted a missing element in row 5 of his group 3. He said it would have an atomic mass of about 68 and be a soft metal like other group 3 elements. Scientists searched for the missing element. They found it a few years later and named it gallium. Scientists searched for the other missing elements. Eventually, all of them were found. An important measure of a good model is its ability to make accurate predictions. This makes it a useful model. Clearly, Mendeleevs periodic table was a useful model. It helped scientists discover new elements and make sense of those that were already known. | text | null |
L_0775 | how elements are organized | T_4000 | A periodic table is still used today to classify the elements. Figure 6.3 shows the modern periodic table. You can see an interactive version at this URL: . | text | null |
L_0775 | how elements are organized | T_4001 | In the modern periodic table, elements are organized by atomic number. The atomic number is the number of protons in an atom of an element. This number is unique for each element, so it seems like an obvious way to organize the elements. (Mendeleev used atomic mass instead of atomic number because protons had not yet been discovered when he made his table.) In the modern table, atomic number increases from left to right across each period. It also increases from top to bottom within each group. How is this like Mendeleevs table? | text | null |
L_0775 | how elements are organized | T_4002 | Besides atomic number, the periodic table includes each elements chemical symbol and class. Some tables include other information as well. The chemical symbol consists of one or two letters that come from the chemicals name in English or another language. The first letter is always written in upper case. The second letter, if there is one, is always written in lower case. For example, the symbol for lead is Pb. It comes from the Latin word plumbum, which means "lead." Find lead in Figure 6.3. What is its atomic number? You can access videos about lead and other elements in the modern periodic table at this URL: . The classes of elements are metals, metalloids, and nonmetals. They are color-coded in the table. Blue stands for metals, orange for metalloids, and green for nonmetals. You can read about each of these three classes of elements later in the chapter, in the lesson "Classes of Elements." | text | null |
L_0775 | how elements are organized | T_4003 | Rows of the modern table are called periods, as they are in Mendeleevs table. From left to right across a period, each element has one more proton than the element before it. In each period, elements change from metals on the left side of the table, to metalloids, and then to nonmetals on the right. Figure 6.4 shows this for period 4. Some periods in the modern periodic table are longer than others. For example, period 1 contains only two elements. Periods 6 and 7, in contrast, are so long that some of their elements are placed below the main part of the table. They are the elements starting with lanthanum (La) in period 6 and actinium (Ac) in period 7. Some elements in period 7 have not yet been named. They are represented by temporary symbols, such as Uub. Many of these elements have only recently been shown to exist. Elements 114 and 116 were added to the table in 2011. Four more elements (113, 115, 117, and 118) were approved for addition in December 2015 and will be named at some later date. | text | null |
L_0775 | how elements are organized | T_4004 | Columns of the modern table are called groups, as they are in Mendeleevs table. However, the modern table has many more groups 18 to be exact. Elements in the same group have similar properties. For example, all elements in group 18 are colorless, odorless gases. You can read about the different groups of elements in this chapters lesson on "Groups of Elements." | text | null |
L_0776 | classes of elements | T_4005 | Metals are elements that are good conductors of electricity. They are the largest of the three classes of elements. In fact, most elements are metals. Look back at the modern periodic table (Figure 6.3) in this chapters lesson "How Elements Are Organized." Find the metals in the table. They are all the elements that are color-coded blue. Examples include sodium (Na), silver (Ag), and zinc (Zn). Metals have relatively high melting points, so almost all are solids at room temperature. The only exception is mercury (Hg), which is a liquid. Most metals are also good conductors of heat. Thats why they are used for cooking pots and stovetops. Metals have other characteristic properties as well. Most are shiny, ductile, and malleable. These properties are illustrated in Figure 6.5. You can dig deeper into the properties of metals at this URL: | text | null |
L_0776 | classes of elements | T_4006 | Nonmetals are elements that do not conduct electricity. They are the second largest class of elements. Find the nonmetals in Figure 6.3. They are all the elements on the right side of the table that are color-coded green. Examples of nonmetals include helium (He), carbon (C), and oxygen (O). Nonmetals generally have properties that are the opposite of those of metals. They also tend to vary more in their properties than metals do. For example, nonmetals have relatively low boiling points, so many of them are gases at room temperature. But several nonmetals are solids, including carbon and phosphorus (P). One nonmetal, bromine (Br), is a liquid at room temperature. Generally, nonmetals are also poor conductors of heat. In fact, they may be used for insulation. For example, the down filling in a down jacket is mostly air, which consists mainly of nitrogen (N) and oxygen (O). These nonmetal gases are poor conductors of heat, so they keep body heat in and cold air out. Solid nonmetals are dull rather than shiny. They are also brittle rather than ductile or malleable. You can see examples of solid nonmetals in Figure 6.6. You can learn more about specific nonmetals with the interactive table at this URL: http://library.thinkquest.org/36 | text | null |
L_0776 | classes of elements | T_4007 | Metalloids are elements that fall between metals and nonmetals in the periodic table. Just seven elements are metalloids, so they are the smallest class of elements. In Figure 6.3, they are color-coded orange. Examples of metalloids include boron (B), silicon (Si), and germanium (Ge). Metalloids have some properties of metals and some properties of nonmetals. For example, many metalloids can conduct electricity but only at certain temperatures. These metalloids are called semiconductors. Silicon is an example. It is used in computer chips. It is also the most common metalloid on Earth. It is shiny like a metal but brittle like a nonmetal. You see a sample of silicon in Figure 6.7. The figure also shows other examples of metalloids. You can learn more about the properties of metalloids at this URL: http://library.thinkquest.org/3659/p | text | null |
L_0776 | classes of elements | T_4008 | From left to right across the periodic table, each element has one more proton than the element to its left. Because atoms are always electrically neutral, for each added proton, one electron is also added. Electrons are added first to the lowest energy level possible until that level is full. Only then are electrons added to the next higher energy level. | text | null |
L_0776 | classes of elements | T_4009 | The increase in electrons across the periodic table explains why elements go from metals to metalloids and then to nonmetals from left to right across the table. Look at period 2 in Figure 6.8 as an example. Lithium (Li) is a metal, boron (B) a metalloid, and fluorine (F) and neon (Ne) are nonmetals. The inner energy level is full for all four elements. This level has just one orbital and can hold a maximum of two electrons. The outer energy level is a different story. This level has four orbitals and can hold a maximum of eight electrons. Lithium has just one electron in this level, boron has three, fluorine has seven, and neon has eight. | text | null |
L_0776 | classes of elements | T_4010 | The electrons in the outer energy level of an atom are called valence electrons. It is valence electrons that are potentially involved in chemical reactions. The number of valence electrons determines an elements reactivity, or how likely the element is to react with other elements. The number of valence electrons also determines whether the element can conduct electric current. Thats because electric current is the flow of electrons. Table 6.1 shows how these properties vary in elements from each class. Metals such as lithium have an outer energy level that is almost empty. They "want" to give up their few valence electrons so they will have a full outer energy level. As a result, metals are very reactive and good conductors of electricity. Metalloids such as boron have an outer energy level that is about half full. These elements need to gain or lose too many electrons for a full outer energy level to come about easily. As a result, these elements are not very reactive. They may be able to conduct electricity but not very well. Some nonmetals, such as bromine, have an outer energy level that is almost full. They "want" to gain electrons so they will have a full outer energy level. As a result, these nonmetals are very reactive. Because they only accept electrons and do not give them up, they do not conduct electricity. Other nonmetals, such as neon, have a completely full outer energy level. Their electrons are already in the most stable arrangement possible. They are unreactive and do not conduct electricity. Element Description Element Lithium Description Lithium (Li) is a highly reactive metal. It has just one electron in its outer energy level. Lithium reacts explosively with water (see picture). It can react with moisture on skin and cause serious burns. Boron Boron (B) is a metalloid. It has three valence electrons and is less reactive than lithium. Boron compounds dissolved in water form boric acid. Dilute boric acid is weak enough to use as eye wash. Bromine Bromine (Br) is an extremely reactive nonmetal. In fact, reactions with fluorine are often explosive, as you can see in the URL below. Neon (Ne) is a nonmetal gas with a completely filled outer energy level. This makes it unreactive, so it doesnt combine with other elements. Neon is used for lighted signs like this one. You can learn why neon gives off light at this link: Neon | text | null |
L_0777 | groups of elements | T_4011 | All the elements in group 1 have just one valence electron, so they are highly reactive. Group 1 is shown in Figure element in the universe. All the other elements in group 1 are alkali metals. They are the most reactive of all metals, and along with the elements in group 17, the most reactive elements. Because alkali metals are so reactive, they are only found in nature combined with other elements. The alkali metals are soft. Most are soft enough to cut with a knife. They are also low in density. Some of them even float on water. All are solids at room temperature. You can see a video demonstrating the reactivity of alkali metals with water at this URL: (2:22). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0777 | groups of elements | T_4012 | The alkaline Earth metals include all the elements in group 2 (see Figure 6.10). These metals have just two valence electrons, so they are very reactive, although not quite as reactive as the alkali metals. In nature, they are always found combined with other elements. Alkaline Earth metals are silvery grey in color. They are harder and denser than the alkali metals. All are solids at room temperature. | text | null |
L_0777 | groups of elements | T_4013 | Groups 3-12 of the periodic table contain transition metals (see Figure 6.11). Transition metals have more valence electrons and are less reactive than metals in the first two metal groups. The transition metals are shiny. Many are silver colored. They tend to be very hard, with high melting and boiling points. All except mercury (Hg) are solids at room temperature. Transition metals include the elements that are placed below the periodic table. Those that follow lanthanum (La) are called lanthanides. They are all shiny, relatively reactive metals. Those that follow Actinium (Ac) are called actinides. They are all radioactive metals. This means they are unstable. They break down into different, more stable elements. You can read more about radioactive elements in the chapter Nuclear Chemistry. Many of the actinides do not occur in nature but are made in laboratories. | text | null |
L_0777 | groups of elements | T_4013 | Groups 3-12 of the periodic table contain transition metals (see Figure 6.11). Transition metals have more valence electrons and are less reactive than metals in the first two metal groups. The transition metals are shiny. Many are silver colored. They tend to be very hard, with high melting and boiling points. All except mercury (Hg) are solids at room temperature. Transition metals include the elements that are placed below the periodic table. Those that follow lanthanum (La) are called lanthanides. They are all shiny, relatively reactive metals. Those that follow Actinium (Ac) are called actinides. They are all radioactive metals. This means they are unstable. They break down into different, more stable elements. You can read more about radioactive elements in the chapter Nuclear Chemistry. Many of the actinides do not occur in nature but are made in laboratories. | text | null |
L_0777 | groups of elements | T_4014 | Groups 13-16 each contain one or more metalloids. These groups are shown in Figure 6.12. Group 13 is called the boron group. The only metalloid in this group is boron (B). The other four elements are metals. All group 13 elements have three valence electrons and are fairly reactive. All are solids at room temperature. Group 14 is called the carbon group. Carbon (C) is a nonmetal. The next two elements are metalloids, and the final two are metals. All the elements in the carbon group have four valence electrons. They are not very reactive. All are solids at room temperature. Group 15 is called the nitrogen group. The first two elements in this group are nonmetals. These are followed by two metalloids and one metal. All the elements in the nitrogen group have five valence electrons, but they vary in their reactivity. Nitrogen (N) in not reactive at all. Phosphorus (P), in contrast, is quite reactive. In fact, it is found naturally only in combination with other substances. Nitrogen is a gas at room temperature. The other group 15 elements are solids. Group 16 is called the oxygen group. The first three elements in this group are nonmetals. They are followed by one metalloid and one metal. All the elements in the oxygen group have six valence electrons, and all are | text | null |
L_0777 | groups of elements | T_4015 | Elements in group 17 are called halogens (see Figure 6.13). They are highly reactive nonmetals with seven valence electrons. The halogens react violently with alkali metals, which have one valence electron. The two elements combine to form a salt. For example, the halogen chlorine (Cl) and the alkali metal sodium (Na) react to form table salt, or sodium chloride (NaCl). The halogen group includes gases, liquids, and solids. For example, chlorine is a gas at room temperature, bromine (Br) is a liquid, and iodine (I) is a solid. You can watch a video demonstrating the reactivity of halogens at this URL: . | text | null |
L_0777 | groups of elements | T_4016 | Group 18 elements are nonmetals called noble gases (see Figure 6.14). They are all colorless, odorless gases. Their outer energy level is also full, so they are the least reactive elements. In nature, they seldom combine with other substances. For a short video about the noble gases and their properties, go to this URL: | text | null |
L_0778 | introduction to chemical bonds | T_4017 | Elements form compounds when they combine chemically. Their atoms join together to form molecules, crystals, or other structures. The atoms are held together by chemical bonds. A chemical bond is a force of attraction between atoms or ions. It occurs when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom. You can learn more about chemical bonds in this video: MEDIA Click image to the left or use the URL below. URL: Look at the example of water in Figure 7.1. A water molecule consists of two atoms of hydrogen and one atom of oxygen. Each hydrogen atom has just one electron. The oxygen atom has six valence electrons. In a water molecule, two hydrogen atoms share their two electrons with the six valence electrons of one oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. This gives it a more stable arrangement of electrons that takes less energy to maintain. | text | null |
L_0778 | introduction to chemical bonds | T_4018 | Water (H2 O) is an example of a chemical compound. Water molecules always consist of two atoms of hydrogen and one atom of oxygen. Like water, all other chemical compounds consist of a fixed ratio of elements. It doesnt matter how much or how little of a compound there is. It always has the same composition. | text | null |
L_0778 | introduction to chemical bonds | T_4019 | Elements are represented by chemical symbols. Examples are H for hydrogen and O for oxygen. Compounds are represented by chemical formulas. Youve already seen the chemical formula for water. Its H2 O. The subscript 2 after the H shows that there are two atoms of hydrogen in a molecule of water. The O for oxygen has no subscript. When there is just one atom of an element in a molecule, no subscript is used. Table 7.1 shows some other examples of compounds and their chemical formulas. Name of Compound Electron Dot Diagram Numbers of Atoms Chemical Formula Name of Compound Hydrogen chloride Electron Dot Diagram Numbers of Atoms H=1 Cl = 1 Chemical Formula HCl Methane C=1 H=4 CH4 Hydrogen peroxide H=2 O=2 H2 O2 Carbon dioxide C=1 O=2 CO2 Problem Solving Problem: A molecule of ammonia consists of one atom of nitrogen (N) and three atoms of hydrogen (H). What is its chemical formula? Solution: The chemical formula is NH3 . You Try It! Problem: A molecule of nitrogen dioxide consists of one atom of nitrogen (N) and two atoms of oxygen (O). What is its chemical formula? | text | null |
L_0778 | introduction to chemical bonds | T_4020 | The same elements may combine in different ratios. If they do, they form different compounds. Figure 7.2 shows some examples. Both water (H2 O) and hydrogen peroxide (H2 O2 ) consist of hydrogen and oxygen. However, they have different ratios of the two elements. As a result, water and hydrogen peroxide are different compounds with different properties. If youve ever used hydrogen peroxide to disinfect a cut, then you know that it is very different from water! Both carbon dioxide (CO2 ) and carbon monoxide (CO) consist of carbon and oxygen, but in different ratios. How do their properties differ? | text | null |
L_0778 | introduction to chemical bonds | T_4021 | There are different types of compounds. They differ in the nature of the bonds that hold their atoms together. The type of bonds in a compound determines many of its properties. Three types of bonds are ionic, covalent, and metallic bonds. You will read about these three types in later lessons. You can also learn more about them by watching this video: (7:18). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0778 | introduction to chemical bonds | T_4022 | Chocolate: Its been revered for millennia by cultures throughout the world. But while its easy to appreciate all of its delicious forms, creating this confection is a complex culinary feat. Local chocolate makers explain the elaborate engineering and chemistry behind this tasty treat. And learn why its actually good for your health! For more information on the science of chocolate, see http://science.kqed.org/quest/video/the-sweet-science-of-chocolate/ . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0779 | ionic bonds | T_4023 | An ionic bond is the force of attraction that holds together positive and negative ions. It forms when atoms of a metallic element give up electrons to atoms of a nonmetallic element. Figure 7.3 shows how this happens. In row 1 of Figure 7.3, an atom of sodium donates an electron to an atom of chlorine (Cl). By losing an electron, the sodium atom becomes a sodium ion. It now has one less electron than protons, giving it a charge of +1. Positive ions such as sodium are given the same name as the element. The chemical symbol has a plus sign to distinguish the ion from an atom of the element. The symbol for a sodium ion is Na+ . By gaining an electron, the chlorine atom becomes a chloride ion. It now has one more electron than protons, giving it a charge of -1. Negative ions are named by adding the suffix ide to the first part of the element name. The symbol for chloride is Cl . Sodium and chloride ions have equal but opposite charges. Opposites attract, so sodium and chloride ions attract each other. They cling together in a strong ionic bond. You can see this in row 2 of Figure 7.3. Brackets separate the ions in the diagram to show that the ions in the compound do not share electrons. You can see animations of sodium chloride forming at these URLs: http://web.jjay.cuny.edu/~acarpi/NSC/salt.htm | text | null |
L_0779 | ionic bonds | T_4024 | Ionic bonds form only between metals and nonmetals. Metals "want" to give up electrons, and nonmetals "want" to gain electrons. Find sodium (Na) in Figure 7.4. Sodium is an alkali metal in group 1. Like other group 1 elements, it has just one valence electron. If sodium loses that one electron, it will have a full outer energy level. Now find fluorine (F) in Figure 7.4. Fluorine is a halogen in group 17. It has seven valence electrons. If fluorine gains one electron, it will have a full outer energy level. After sodium gives up its valence electron to fluorine, both atoms have a more stable arrangement of electrons. | text | null |
L_0779 | ionic bonds | T_4025 | It takes energy to remove valence electrons from an atom. The force of attraction between the negative electrons and positive nucleus must be overcome. The amount of energy needed depends on the element. Less energy is needed to remove just one or a few electrons than many. This explains why sodium and other alkali metals form positive ions so easily. Less energy is also needed to remove electrons from larger atoms in the same group. For example, in group 1, it takes less energy to remove an electron from francium (Fr) at the bottom of the group than from lithium (Li) at the top of the group (see Figure 7.4). In bigger atoms, valence electrons are farther from the nucleus. As a result, the force of attraction between the electrons and nucleus is weaker. What happens when an atom gains an electron and becomes a negative ion? Energy is released. Halogens release the most energy when they form ions. As a result, they are very reactive. | text | null |
L_0779 | ionic bonds | T_4026 | Ionic compounds contain ions of metals and nonmetals held together by ionic bonds. Ionic compounds do not form molecules. Instead, many positive and negative ions bond together to form a structure called a crystal. You can see an example of a crystal in Figure 7.5. It shows the ionic compound sodium chloride. Positive sodium ions (Na+ ) alternate with negative chloride ions (Cl ). The oppositely charged ions are strongly attracted to each other. Helpful Hints Naming Ionic Compounds Ionic compounds are named for their positive and negative ions. The name of the positive | text | null |
L_0779 | ionic bonds | T_4027 | The crystal structure of ionic compounds is strong and rigid. It takes a lot of energy to break all those strong ionic bonds. As a result, ionic compounds are solids with high melting and boiling points (see Table 7.2). The rigid crystals are brittle and more likely to break than bend when struck. As a result, ionic crystals tend to shatter. You can learn more about the properties of ionic compounds by watching the video at this URL: MEDIA Click image to the left or use the URL below. URL: Compare the melting and boiling points of these ionic compounds with those of water (0C and 100C), which is not an ionic compound. Ionic Compound Sodium chloride (NaCl) Calcium chloride (CaCl2 ) Barium oxide (BaO) Iron bromide (FeBr3 ) Melting Point (C) 801 772 1923 684 Boiling Point (C) 1413 1935 2000 934 Solid ionic compounds are poor conductors of electricity. The strong bonds between ions lock them into place in the crystal. However, in the liquid state, ionic compounds are good conductors of electricity. Most ionic compounds dissolve easily in water. When they dissolve, they separate into individual ions. The ions can move freely, so they are good conductors of electricity. Dissolved ionic compounds are called electrolytes. | text | null |
L_0779 | ionic bonds | T_4028 | Ionic compounds have many uses. Some are shown in Figure 7.6. Many ionic compounds are used in industry. The human body also needs several ions for good health. Having low levels of the ions can endanger important functions such as heartbeat. Solutions of ionic compounds can be used to restore the ions. | text | null |
L_0780 | covalent bonds | T_4029 | A covalent bond is the force of attraction that holds together two atoms that share a pair of electrons. The shared electrons are attracted to the nuclei of both atoms. Covalent bonds form only between atoms of nonmetals. The two atoms may be the same or different elements. If the bonds form between atoms of different elements, a covalent compound forms. Covalent compounds are described in detail later in the lesson. To see a video about covalent bonding, go to this URL: (6:20). MEDIA Click image to the left or use the URL below. URL: Figure 7.7 shows an example of a covalent bond forming between two atoms of the same element, in this case two atoms of hydrogen. The two atoms share a pair of electrons. Hydrogen normally occurs in two-atom, or diatomic, molecules like this (di- means "two"). Several other elements also normally occur as diatomic molecules: nitrogen, oxygen, and all but one of the halogens (fluorine, chlorine, bromine, and iodine). | text | null |
L_0780 | covalent bonds | T_4030 | Covalent bonds form because they give atoms a more stable arrangement of electrons. Look at the hydrogen atoms in Figure 7.7. Alone, each hydrogen atom has just one electron. By sharing electrons with another hydrogen atom, it has two electrons: its own and the one in the other hydrogen atom. The shared electrons are attracted to both hydrogen nuclei. This force of attraction holds the two atoms together as a molecule of hydrogen. Some atoms need to share more than one pair of electrons to have a full outer energy level. For example, an oxygen atom has six valence electrons. It needs two more electrons to fill its outer energy level. Therefore, it must form two covalent bonds. This can happen in many different ways. One way is shown in Figure 7.8. The oxygen atom in the figure has covalent bonds with two hydrogen atoms. This forms the covalent compound water. | text | null |
L_0780 | covalent bonds | T_4031 | In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. Figure 7.9 shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. For another example of polar bonds, see the video at this URL: (0:52). MEDIA Click image to the left or use the URL below. URL: In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral. Figure 7.10 shows an example of nonpolar bonds. | text | null |
L_0780 | covalent bonds | T_4031 | In some covalent bonds, electrons are not shared equally between the two atoms. These are called polar bonds. Figure 7.9 shows this for water. The oxygen atom attracts the shared electrons more strongly because its nucleus has more positively charged protons. As a result, the oxygen atom becomes slightly negative in charge. The hydrogen atoms attract the electrons less strongly. They become slightly positive in charge. For another example of polar bonds, see the video at this URL: (0:52). MEDIA Click image to the left or use the URL below. URL: In other covalent bonds, electrons are shared equally. These bonds are called nonpolar bonds. Neither atom attracts the shared electrons more strongly. As a result, the atoms remain neutral. Figure 7.10 shows an example of nonpolar bonds. | text | null |
L_0780 | covalent bonds | T_4032 | Covalent bonds between atoms of different elements form covalent compounds. The smallest, simplest covalent compounds have molecules with just two atoms. An example is hydrogen chloride (HCl). It consists of one hydrogen atom and one chlorine atom. The largest, most complex covalent molecules have thousands of atoms. Examples include proteins and carbohydrates. These are compounds in living things. Helpful Hints Naming Covalent Compounds Follow these rules in naming simple covalent compounds: The element closer to the left of the periodic table is named first. The second element gets the suffix ide. Prefixes such as di- (2) and tri- (3) show the number of each atom in the compound. These are written with subscripts in the chemical formula. Example: The gas that consists of one carbon atom and two oxygen atoms is named carbon dioxide. Its chemical formula is CO2 . You Try It! Problem: What is the name of the compound that contains three oxygen atoms and two nitrogen atoms? What is its chemical formula? | text | null |
L_0780 | covalent bonds | T_4033 | Covalent compounds have different properties than ionic compounds because of their bonds. Covalent compounds exist as individual molecules rather than crystals. It takes less energy for individual molecules than ions in a crystal to pull apart. As a result, covalent compounds have lower melting and boiling points than ionic compounds. Many are gases or liquids at room temperature. Covalent compounds have shared electrons. These are not free to move like the transferred electrons of ionic compounds. This makes covalent compounds poor conductors of electricity. Many covalent compounds also do not dissolve in water as all ionic compounds do. | text | null |
L_0780 | covalent bonds | T_4034 | Having polar bonds may make a covalent compound polar. A polar compound is one in which there is a slight difference in charge between opposite ends of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in Figure 7.11. Both molecules in the figure contain polar bonds, but only formaldehyde is a polar compound. Why is carbon dioxide nonpolar? The molecules of polar compounds are attracted to each other. You can see this in Figure 7.12 for water. A bond forms between the positive hydrogen end of one water molecule and the negative oxygen end of another water molecule. This type of bond is called a hydrogen bond. Hydrogen bonds are weak, but they still must be overcome when a polar substance changes from a solid to a liquid or from a liquid to a gas. As a result, polar covalent compounds may have higher melting and boiling points than nonpolar covalent compounds. To learn more about hydrogen bonding and when it occurs, see the video at this URL: (0:58). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0780 | covalent bonds | T_4034 | Having polar bonds may make a covalent compound polar. A polar compound is one in which there is a slight difference in charge between opposite ends of the molecule. All polar compounds contain polar bonds. But having polar bonds does not necessarily result in a polar compound. It depends on how the atoms are arranged. This is illustrated in Figure 7.11. Both molecules in the figure contain polar bonds, but only formaldehyde is a polar compound. Why is carbon dioxide nonpolar? The molecules of polar compounds are attracted to each other. You can see this in Figure 7.12 for water. A bond forms between the positive hydrogen end of one water molecule and the negative oxygen end of another water molecule. This type of bond is called a hydrogen bond. Hydrogen bonds are weak, but they still must be overcome when a polar substance changes from a solid to a liquid or from a liquid to a gas. As a result, polar covalent compounds may have higher melting and boiling points than nonpolar covalent compounds. To learn more about hydrogen bonding and when it occurs, see the video at this URL: (0:58). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0781 | metallic bonds | T_4035 | A metallic bond is the force of attraction between a positive metal ion and the valence electrons it shares with other ions of the metal. The positive ions form a lattice-like structure. You can see an example in Figure 7.13. (For an animated version, go to the URL below.) The ions are held together in the lattice by bonds with the valence electrons around them. These valence electrons include their own and those of other ions. Why do metallic bonds form? Recall that metals "want" to give up their valence electrons. This means that their valence electrons move freely. The electrons form a "sea" of negative charge surrounding the positive ions. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0781 | metallic bonds | T_4036 | Because of their freely moving electrons, metals are good conductors of electricity. Metals also can be shaped without breaking. They are ductile (can be shaped into wires) and malleable (can be shaped into thin sheets). Metals have these properties because of the nature of their metallic bonds. A metallic lattice, like the one in Figure 7.13, may resemble a rigid ionic crystal. However, it is much more flexible. Look at Figure 7.14. It shows a blacksmith hammering a piece of red-hot iron in order to shape it. Why doesnt the iron shatter, as an ionic crystal would? The ions of the metal can move within the "sea" of electrons without breaking the metallic bonds that hold them together. The ions can shift closer together or farther apart. In this way, the metal can change shape without breaking. You can learn more about metallic bonds and the properties of metals at this URL: (6:12). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0781 | metallic bonds | T_4037 | Metals are useful for many purposes because of their unique properties. However, pure metals may be less useful than mixtures of metals. For example, iron is not as strong as steel, which is a mixture of iron and small amounts of carbon. Steel is so strong that it can hold up huge bridges, like the one Figure 7.15. Steel is also used to make skyscrapers, cargo ships, cars, and trains. Steel is an example of an alloy. An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is a solid solution. It is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Several other examples of alloys and their uses are shown in Figure 7.16. You can learn about an amazing alloy called memory wire at the URL below. If you have braces on your teeth, you may even have this alloy in your mouth! | text | null |
L_0781 | metallic bonds | T_4037 | Metals are useful for many purposes because of their unique properties. However, pure metals may be less useful than mixtures of metals. For example, iron is not as strong as steel, which is a mixture of iron and small amounts of carbon. Steel is so strong that it can hold up huge bridges, like the one Figure 7.15. Steel is also used to make skyscrapers, cargo ships, cars, and trains. Steel is an example of an alloy. An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is a solid solution. It is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Several other examples of alloys and their uses are shown in Figure 7.16. You can learn about an amazing alloy called memory wire at the URL below. If you have braces on your teeth, you may even have this alloy in your mouth! | text | null |
L_0782 | introduction to chemical reactions | T_4038 | A chemical reaction is a process in which some substances change into different substances. Substances that start a chemical reaction are called reactants. Substances that are produced in the reaction are called products. Reactants and products can be elements or compounds. A chemical reaction can be represented by this general equation: Reactants ! Products The arrow (!) shows the direction in which the reaction occurs. The reaction may occur quickly or slowly. For example, foam shoots out of a fire extinguisher as soon as the lever is pressed. But it might take years for metal to rust. | text | null |
L_0782 | introduction to chemical reactions | T_4039 | In chemical reactions, bonds break in the reactants and new bonds form in the products. The reactants and prod- ucts contain the same atoms, but they are rearranged during the reaction. As a result, the atoms are in different combinations in the products than they were in the reactants. Look at the example in Figure 8.2. It shows how water forms. Bonds break in molecules of hydrogen and oxygen. Then new bonds form in molecules of water. In both reactants and products, there are four hydrogen atoms and two oxygen atoms. But the atoms are combined differently in water. You can see another example at this URL: http://w | text | null |
L_0782 | introduction to chemical reactions | T_4040 | The arrow in Figure 8.2 shows that the reaction goes from left to right, from hydrogen and oxygen to water. The reaction can also go in the reverse direction. If an electric current passes through water, water molecules break down into molecules of hydrogen and oxygen. This reaction would be represented by a right-to-left arrow ( ) in Figure Many other reactions can also go in both forward and reverse directions. Often, a point is reached at which the forward and reverse reactions occur at the same rate. When this happens, there is no overall change in the amount of reactants and products. This point is called equilibrium, which refers to a balance between any opposing changes. You can see an animation of a chemical reaction reaching equilibrium at this URL: | text | null |
L_0782 | introduction to chemical reactions | T_4041 | Not all changes in matter involve chemical reactions. For example, there are no chemical reactions involved in changes of state. When liquid water freezes or evaporates, it is still water. No bonds are broken and no new products are formed. How can you tell whether a change in matter involves a chemical reaction? Often, there is evidence. Four common signs that a chemical reaction has occurred are: Change in color: the products are a different color than the reactants. Change in temperature: heat is released or absorbed during the reaction. Production of a gas: gas bubbles are released during the reaction. Production of a solid: a solid settles out of a liquid solution. The solid is called a precipitate. You can see examples of each type of evidence in Figure 8.3 and at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0783 | chemical equations | T_4042 | A chemical equation is a symbolic representation of a chemical reaction. It is a shorthand way of showing how atoms are rearranged in the reaction. The general form of a chemical equation was introduced in this chapters lesson "Introduction to Chemical Reactions." It is: Reactants ! Products Consider the simple example in Figure 8.4. When carbon (C) reacts with oxygen (O2 ), it produces carbon dioxide (CO2 ). The chemical equation for this reaction is: C + O2 ! CO2 The reactants are one atom of carbon and one molecule of oxygen. When there is more than one reactant, they are separated by plus signs (+). The product is one molecule of carbon dioxide. If more than one product were produced, plus signs would be used between them as well. | text | null |
L_0783 | chemical equations | T_4043 | Some chemical equations are more challenging to write. Consider the reaction in which hydrogen (H2 ) and oxygen (O2 ) combine to form water (H2 O). Hydrogen and oxygen are the reactants, and water is the product. To write a chemical equation for this reaction, you would start by writing symbols for the reactants and products: Equation 1: H2 + O2 ! H2 O Like equations in math, equations in chemistry must balance. There must be the same number of each type of atom in the products as there is in the reactants. In equation 1, count the number of hydrogen and oxygen atoms on each side of the arrow. There are two hydrogen atoms in both reactants and products. There are two oxygen atoms in the reactants but only one in the product. Therefore, equation 1 is not balanced. | text | null |
L_0783 | chemical equations | T_4044 | Coefficients are used to balance chemical equations. A coefficient is a number placed in front of a chemical symbol or formula. It shows how many atoms or molecules of the substance are involved in the reaction. For example, two molecules of hydrogen would be written as 2H2 . A coefficient of 1 usually isnt written. Coefficients can be used to balance equation 1 (above) as follows: Equation 2: 2H2 + O2 ! 2H2 O Equation 2 shows that two molecules of hydrogen react with one molecule of oxygen to produce two molecules of water. The two molecules of hydrogen each contain two hydrogen atoms. There are now four hydrogen atoms in both reactants and products. Is equation 2 balanced? Count the oxygen atoms to find out. | text | null |
L_0783 | chemical equations | T_4045 | Balancing a chemical equation involves a certain amount of trial and error. In general, however, you should follow these steps: 1. Count the number of each type of atom in reactants and products. Does the same number of each atom appear on both sides of the arrow? If not, the equation is not balanced, and you need to go to step 2. 2. Add coefficients to increase the number of atoms or molecules of reactants or products. Use the smallest coefficients possible. 3. Repeat steps 1 and 2 until the equation is balanced. Helpful Hint When you balance chemical equations, never change the subscripts in chemical formulas. Changing subscripts changes the substances involved in the reaction. Change only the coefficients. Work through the Problem Solving examples below. Then do the You Try It! problems to check your understand- ing. If you need more help, go to this URL: (14:28). MEDIA Click image to the left or use the URL below. URL: Problem Solving Problem: Balance this chemical equation: N2 + H2 ! NH3 Hints for balancing 1. Two N are needed in the products to match the two N (N2 ) in the reactants. Add the coefficient 2 in front of NH3 . Now N is balanced. 2. Six H are now needed in the reactants to match the six H in the products. Add the coefficient 3 in front of H2 . Now H is balanced. Solution: N2 + 3H2 ! 2NH3 Problem: Balance this chemical equation: CH4 + O2 ! CO2 + H2 O Solution: CH4 + 2O2 ! CO2 + 2H2 O You Try It! Problem: Balance these chemical equations: Zn + HCl ! ZnCl2 + H2 Cu + O2 ! CuO | text | null |
L_0783 | chemical equations | T_4046 | Why must chemical equations be balanced? Its the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants. Balanced chemical equations show that mass is conserved in chemical reactions. How do scientists know that mass is always conserved in chemical reactions? Careful experiments in the 1700s by a French chemist named Antoine Lavoisier led to this conclusion. For this and other contributions, Lavoisier has been called the father of modern chemistry. Lavoisier carefully measured the mass of reactants and products in many different chemical reactions. He carried out the reactions inside a sealed jar, like the one in Figure 8.5. As a result, any gases involved in the reactions were captured and could be measured. In every case, the total mass of the jar and its contents was the same after the reaction as it was before the reaction took place. This showed that matter was neither created nor destroyed in the reactions. Another outcome of Lavoisiers research was his discovery of oxygen. You can learn more about Lavoisier and his important research at: | text | null |
L_0784 | types of chemical reactions | T_4047 | A synthesis reaction occurs when two or more reactants combine to form a single product. A synthesis reaction can be represented by the general equation: A+B !C In this general equation (and others like it in this lesson), the letters A, B,C, and so on represent atoms or ions of elements. The arrow shows the direction of the reaction. The letters on the left side of the arrow are the reactants that begin the chemical reaction. The letters on the right side of the arrow are the product of the reaction. Two examples of synthesis reactions are described below. You can see more examples at this URL: | text | null |
L_0784 | types of chemical reactions | T_4048 | An example of a synthesis reaction is the combination of sodium (Na) and chlorine (Cl) to produce sodium chloride (NaCl). This reaction is represented by the chemical equation: 2Na + Cl2 ! 2NaCl Sodium is a highly reactive metal, and chlorine is a poisonous gas (see Figure 8.6). The compound they synthesize has very different properties. It is table salt, which is neither reactive nor poisonous. In fact, salt is a necessary component of the human diet. | text | null |
L_0784 | types of chemical reactions | T_4049 | Another example of a synthesis reaction is illustrated in Figure 8.7. The brown haze in the air over the city of Los Angeles is smog. A major component of smog is nitrogen dioxide (NO2 ). It forms when nitric oxide (NO), from sources such as car exhaust, combines with oxygen (O2 ) in the air. The equation for this reaction is: 2NO + O2 ! 2NO2 Nitrogen dioxide is a toxic gas with a sharp odor. It can irritate the eyes and throat and trigger asthma attacks. It is a major air pollutant. | text | null |
L_0784 | types of chemical reactions | T_4050 | A decomposition reaction is the reverse of a synthesis reaction. In a decomposition reaction, one reactant breaks down into two or more products. This can be represented by the general equation: AB ! A + B Two examples of decomposition reactions are described below. You can see other examples at this URL: http://w | text | null |
L_0784 | types of chemical reactions | T_4051 | An example of a decomposition reaction is the breakdown of carbonic acid (H2 CO3 ) to produce water (H2 O) and carbon dioxide (CO2 ). The equation for this reaction is: H2 CO3 ! H2 O + CO2 Carbonic acid is synthesized in the reverse reaction. It forms when carbon dioxide dissolves in water. For example, some of the carbon dioxide in the atmosphere dissolves in the ocean and forms carbonic acid. The amount of carbon dioxide in the atmosphere has increased over recent decades (see Figure 8.8). As a result, the acidity of ocean water is also increasing. How do you think this might affect ocean life? | text | null |
L_0784 | types of chemical reactions | T_4052 | Another example of a decomposition reaction is illustrated in Figure 8.9. Water (H2 O) decomposes to hydrogen (H2 ) and oxygen (O2 ) when an electric current passes through it. This reaction is represented by the equation: 2H2 O ! 2H2 + O2 What is the reverse of this decomposition reaction? (Hint: How is water synthesized? You can look at this chapters "Introduction to Chemical Reactions" lesson to find out.) | text | null |
L_0784 | types of chemical reactions | T_4053 | Replacement reactions involve ions. They occur when ions switch places in compounds. There are two types of replacement reactions: single and double. Both types are described below. | text | null |
L_0784 | types of chemical reactions | T_4054 | A single replacement reaction occurs when one ion takes the place of another in a single compound. This type of reaction has the general equation: A + BC ! B + AC Do you see how A has replaced B in the compound? The compound BC has become the compound AC. An example of a single replacement reaction occurs when potassium (K) reacts with water (H2 O). A colorless solid called potassium hydroxide (KOH) forms, and hydrogen gas (H2 ) is released. The equation for the reaction is: 2K + 2H2 O ! 2KOH + H2 Potassium is a highly reactive group 1 alkali metal, so its reaction with water is explosive. You can actually watch this reaction occurring at: http://commons.wikimedia.org/wiki/File:Potassium_water_20.theora.ogv . | text | null |
L_0784 | types of chemical reactions | T_4055 | A double replacement reaction occurs when two compounds exchange ions. This produces two new compounds. A double replacement reaction can be represented by the general equation: AB +CD ! AD +CB Do you see how B and D have changed places? Both reactant compounds have changed. An example of a double replacement reaction is sodium chloride (NaCl) reacting with silver fluoride (AgF). This reaction is represented by the equation: NaCl + AgF ! NaF + AgCl Cl and F have changed places. Can you name the products of this reaction? | text | null |
L_0784 | types of chemical reactions | T_4056 | A combustion reaction occurs when a substance reacts quickly with oxygen (O2 ). You can see an example of a combustion reaction in Figure 8.10. Combustion is commonly called burning. The substance that burns is usually referred to as fuel. The products of a combustion reaction include carbon dioxide (CO2 ) and water (H2 O). The reaction typically gives off heat and light as well. The general equation for a combustion reaction can be represented by: Fuel + O2 ! CO2 + H2 O | text | null |
L_0784 | types of chemical reactions | T_4057 | The fuel that burns in a combustion reaction is often a substance called a hydrocarbon. A hydrocarbon is a compound that contains only carbon (C) and hydrogen (H). Fossil fuels, such as natural gas, consist of hydrocarbons. Natural gas is a fuel that is commonly used in home furnaces and gas stoves (see Figure 8.11). The main component of natural gas is the hydrocarbon called methane (CH4 ). The combustion of methane is represented by the equation: CH4 + 2O2 ! CO2 + 2H2 O | text | null |
L_0784 | types of chemical reactions | T_4058 | Your own body cells burn fuel in combustion reactions. The fuel is glucose (C6 H12 O6 ), a simple sugar. The process in which combustion of glucose occurs in body cells is called cellular respiration. This combustion reaction provides energy for life processes. Cellular respiration can be summed up by the equation: C6 H12 O6 + 6O2 ! 6CO2 + 6H2 O Where does glucose come from? It is produced by plants during photosynthesis. In this process, carbon dioxide and water combine to form glucose. Which type of chemical reaction is photosynthesis? | text | null |
L_0785 | chemical reactions and energy | T_4059 | In an endothermic reaction, it takes more energy to break bonds in the reactants than is released when new bonds form in the products. The word "endothermic" literally means "taking in heat." A constant input of energy, often in the form of heat, is needed in an endothermic reaction. Not enough energy is released when products form to break more bonds in the reactants. Additional energy is needed to keep the reaction going. The general equation for an endothermic reaction is: Reactants + Energy ! Products In many endothermic reactions, heat is absorbed from the surroundings. As a result, the temperature drops. The drop in temperature may be great enough to cause liquid products to freeze. Thats what happens in the endothermic reaction at this URL: One of the most important endothermic reactions is photosynthesis. In this reaction, plants synthesize glucose (C6 H12 O6 ) from carbon dioxide (CO2 ) and water (H2 O). They also release oxygen (O2 ). The energy for photo- synthesis comes from light (see Figure 8.12). Without light energy, photosynthesis cannot occur. The chemical equation for photosynthesis is: 6CO2 + 6H2 O ! C6 H12 O6 + 6O2 | text | null |
L_0785 | chemical reactions and energy | T_4060 | In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products. The word "exothermic" literally means "turning out heat." Energy, often in the form of heat, is released as an exothermic reaction occurs. The general equation for an exothermic reaction is: Reactants ! Products + Energy If the energy is released as heat, an exothermic reaction results in a rise in temperature. Thats what happens in the exothermic reaction at the URL below. Combustion reactions are examples of exothermic reactions. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in Figure 8.13. You can see the light energy it is giving off. If you were standing near the fire, you would also feel its heat. | text | null |
L_0785 | chemical reactions and energy | T_4061 | Whether a reaction absorbs energy or releases energy, there is no overall change in the amount of energy. Energy cannot be created or destroyed. This is the law of conservation of energy. Energy can change form for example, from electricity to light but the same amount of energy always remains. If energy cannot be destroyed, what happens to the energy that is absorbed in an endothermic reaction? The energy is stored in the chemical bonds of the products. This form of energy is called chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy in the reactants is released to the surroundings when the reaction occurs. The graphs in Figure 8.14 show the chemical energy of reactants and products in each type of reaction. | text | null |
L_0785 | chemical reactions and energy | T_4061 | Whether a reaction absorbs energy or releases energy, there is no overall change in the amount of energy. Energy cannot be created or destroyed. This is the law of conservation of energy. Energy can change form for example, from electricity to light but the same amount of energy always remains. If energy cannot be destroyed, what happens to the energy that is absorbed in an endothermic reaction? The energy is stored in the chemical bonds of the products. This form of energy is called chemical energy. In an endothermic reaction, the products have more stored chemical energy than the reactants. In an exothermic reaction, the opposite is true. The products have less stored chemical energy than the reactants. The excess energy in the reactants is released to the surroundings when the reaction occurs. The graphs in Figure 8.14 show the chemical energy of reactants and products in each type of reaction. | text | null |
L_0785 | chemical reactions and energy | T_4062 | All chemical reactions, even exothermic reactions, need a certain amount of energy to get started. This energy is called activation energy. For example, activation energy is needed to start a car. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas wont occur without the spark of energy to begin the reaction. Why is activation energy needed? A reaction wont occur unless atoms or molecules of reactants come together. This happens only if the particles are moving, and movement takes energy. Often, reactants have to overcome forces that push them apart. This takes energy as well. Still more energy is needed to start breaking bonds in reactants. The graphs in Figure 8.15 show the changes in energy in endothermic and exothermic reactions. Both reactions need the same amount of activation energy in order to begin. You have probably used activation energy to start a chemical reaction. For example, if youve ever used a match to light a campfire, then you provided the activation energy needed to start a combustion reaction. Combustion is exothermic. Once a fire starts to burn, it releases enough energy to activate the next reaction, and the next, and so on. However, wood will not burst into flames on its own. | text | null |
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