FIGURE 2.3 The three most abundant isotopes of carbon. Isotopes Carbon -12 of a particular atom have different numbers of 6 Neutro 6 Electrons Isotopes tential to severely damage living cells, producing mutations in Atoms with the same atom nic number(that is, the same num- their genes, and, at high doses, cell death. Consequently,ex osure to radiation is now very carefully controlled and ber of protons) have the same chemical properties and are iated. Scientists who work with radioactivity(basic re- said to belong to the same element. Formally speaking, an element is any substance that cannot be broken down to any searchers as well as applied scientists such as X-ray other substance by ordinary chemical means. However, while technologists)wear radiation-sensitive badges to monitor the all atoms of an element have the same number of protons total amount of radioactivity to which they are exposed. Each they may not all have the same number of neutrons. Atoms of month the badges are collected and scrutinized. Thus, em an element that possess different numbers of neutrons are ployees whose work places them in danger of excessive radio- active exposure are equipped with an"earl ng system as mixtures of different isotopes. Carbon (C), for example, has three isotopes, all containing six protons(figure 2.3) Electrons Over 99% of the carbon found in nature exists as an isotope The positive charges in the nucleus of an atom are counter with six neutrons. Because its total mass is 12 daltons(6 from balanced by negatively charged electrons orbiting at vary- protons plus 6 from neutrons), this isotope is referred to as ing distances around the nucleus. Thus, atoms with the carbon-12, and symbolized 2C. Most of the rest of the natu- same number of protons and electrons are electrically neu- ally occurring carbon is carbon-13 tope with seven tral, having no net ch neutrons. The rarest carbon isotope is carbon-14, with eight Electrons are maintained in their orbits by their attrac neutrons. Unlike the other two isotopes, carbon-14 is unsta- tion to the positively charged nucleus. Sometimes other ble: its nucleus tends to break up into elements with lower forces overcome this attraction and an atom loses one or atomic numbers. This nuclear breakup, which emits a signifi more electrons In other cases, atoms may gain additional cant amount of energy, is called radioactive decay, and iso- electrons. Atoms in which the number of electrons does pes that decay in this fashion are radioactive isotopes not equal the number of protons are known as ions, and however, the rate of decay is constant. This rate is usually called a cation. For example, an atom of sodium(Na)that expressed as the half-life the time it takes for one half of the has lost one electron becomes a sodium ion(Na*), with a atoms in a sample to decay. Carbon-14, for example, has a charge of +1. An atom that has fewer protons than elec half-life of about 5600 years. A sample of carbon containing rons carries a net negative charge and is called an anion. a I gram of carbon-14 today would contain 0.5 gram of car chlorine atom( Ci)that has gained one electron becomes a bon-14 after 5600 years, 0.25 gram 11, 200 years from now, chloride ion( Ch), with a charge of-1 0.125 gram 16, 800 years from now, and so on. By determin ing the ratios of the different isotopes of carbon and other An atom consists of a nucleus of protons and neutrons elements in biological samples and in rocks, scientists ar surrounded by a cloud of electrons. The number of its able to accurately determine when these materials formed electrons largely determines the chemical properties of While there are many useful applications of radioactivity an atom. Atoms that have the same number of protons there are also harmful side effects that must be considered in but different numbers of neutrons are called isotopes any planned use of radioactive substances. Radioactive sub- Isotopes of an atom differ in atomic mass but have stances emit energetic subatomic particles that have the po- similar chemical properties Chapter2 The Nature of Molecules 21Isotopes Atoms with the same atomic number (that is, the same num￾ber of protons) have the same chemical properties and are said to belong to the same element. Formally speaking, an element is any substance that cannot be broken down to any other substance by ordinary chemical means. However, while all atoms of an element have the same number of protons, they may not all have the same number of neutrons. Atoms of an element that possess different numbers of neutrons are called isotopes of that element. Most elements in nature exist as mixtures of different isotopes. Carbon (C), for example, has three isotopes, all containing six protons (figure 2.3). Over 99% of the carbon found in nature exists as an isotope with six neutrons. Because its total mass is 12 daltons (6 from protons plus 6 from neutrons), this isotope is referred to as carbon-12, and symbolized 12C. Most of the rest of the natu￾rally occurring carbon is carbon-13, an isotope with seven neutrons. The rarest carbon isotope is carbon-14, with eight neutrons. Unlike the other two isotopes, carbon-14 is unsta￾ble: its nucleus tends to break up into elements with lower atomic numbers. This nuclear breakup, which emits a signifi￾cant amount of energy, is called radioactive decay, and iso￾topes that decay in this fashion are radioactive isotopes. Some radioactive isotopes are more unstable than others and therefore decay more readily. For any given isotope, however, the rate of decay is constant. This rate is usually expressed as the half-life, the time it takes for one half of the atoms in a sample to decay. Carbon-14, for example, has a half-life of about 5600 years. A sample of carbon containing 1 gram of carbon-14 today would contain 0.5 gram of car￾bon-14 after 5600 years, 0.25 gram 11,200 years from now, 0.125 gram 16,800 years from now, and so on. By determin￾ing the ratios of the different isotopes of carbon and other elements in biological samples and in rocks, scientists are able to accurately determine when these materials formed. While there are many useful applications of radioactivity, there are also harmful side effects that must be considered in any planned use of radioactive substances. Radioactive sub￾stances emit energetic subatomic particles that have the po￾tential to severely damage living cells, producing mutations in their genes, and, at high doses, cell death. Consequently, ex￾posure to radiation is now very carefully controlled and regu￾lated. Scientists who work with radioactivity (basic re￾searchers as well as applied scientists such as X-ray technologists) wear radiation-sensitive badges to monitor the total amount of radioactivity to which they are exposed. Each month the badges are collected and scrutinized. Thus, em￾ployees whose work places them in danger of excessive radio￾active exposure are equipped with an “early warning system.” Electrons The positive charges in the nucleus of an atom are counter￾balanced by negatively charged electrons orbiting at vary￾ing distances around the nucleus. Thus, atoms with the same number of protons and electrons are electrically neu￾tral, having no net charge. Electrons are maintained in their orbits by their attrac￾tion to the positively charged nucleus. Sometimes other forces overcome this attraction and an atom loses one or more electrons. In other cases, atoms may gain additional electrons. Atoms in which the number of electrons does not equal the number of protons are known as ions, and they carry a net electrical charge. An atom that has more protons than electrons has a net positive charge and is called a cation. For example, an atom of sodium (Na) that has lost one electron becomes a sodium ion (Na+), with a charge of +1. An atom that has fewer protons than elec￾trons carries a net negative charge and is called an anion. A chlorine atom (Cl) that has gained one electron becomes a chloride ion (Cl–), with a charge of –1. An atom consists of a nucleus of protons and neutrons surrounded by a cloud of electrons. The number of its electrons largely determines the chemical properties of an atom. Atoms that have the same number of protons but different numbers of neutrons are called isotopes. Isotopes of an atom differ in atomic mass but have similar chemical properties. Chapter 2 The Nature of Molecules 21 Carbon-12 6 Protons 6 Neutrons 6 Electrons Carbon-13 6 Protons 7 Neutrons 6 Electrons Carbon-14 6 Protons 8 Neutrons 6 Electrons FIGURE 2.3 The three most abundant isotopes of carbon. Isotopes of a particular atom have different numbers of neutrons