![]() The terms atomic weight and molecular weight are universally used by working scientists, and will be used in this book, even though these are technically masses rather than weights. Nuclei that have too many of either kind of fundamental particle are unstable, and break down radioactively in ways that are discussed in Chapter 23. The more protons, the greater the ratio of neutrons to protons to ensure stability. ![]() For a nucleus to be stable, the number of neutrons should (for the first few elements) equal or slightly exceed the number of protons. In discussing these isotopes, we use the. Hence the isotope symbols are usually written without the subscript: 35Cl and 37Cl. Strictly speaking, the subscript is unnecessary, since all atoms of chlorine have 17 protons. The symbols for the two naturally occurring isotopes of chlorine then would be Cl and Cl. ![]() To write the symbol for an isotope, place the atomic number as a subscript and the mass number (protons plus neutrons) as a superscript to the left of the atomic symbol. Only two chlorine isotopes exist in significant amounts in nature, those with 18 neutrons (75.53% of all chlorine atoms found in nature), and those with 20 neutrons (24.47%). All atoms of chlorine (Cl) have 17 protons, but there are chlorine isotopes having 15 to 23 neutrons. Four isotopes of helium (He) are shown in Figure 1-1. These differing atoms of the same element are called isotopes. \)Īlthough all atoms of an element have the same number of protons, the atoms may differ in the number of neutrons they have (Table 1-2).
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