The atomic weights given inside the back cover of this book are all weighted averages of the isotopes occurring in nature, and these are the figures we shall use henceforth-unless we are specifically discussing one isotope. Chlorine occurs in nature as 75.53% chlorine-35 (34.97 amu) and 24.47% chlorine-37 (36.97 amu), so the weighted average of the isotope weights is The average is weighted according to the percent abundance of the isotopes. (For example, the atomic weight of chlorine-37 is 36.966, which is rounded to 37.) If there are several isotopes of an element in nature, then of course the experimentally observed atomic weight (the natural atomic weight) will be the weighted average of the isotope weights. Since mass losses upon formation of an atom are small, the mass number is usually the same as the atomic weight rounded to the nearest integer. The chlorine isotope has 17 protons and 20 neutrons: Protons:Įach isotope of an element is characterized by an atomic number (total number of protons), a mass number (total number of protons and neutrons), and an atomic weight (mass of atom in atomic mass units). Compare this with the actual atomic weight of this isotope as given in Table 1-2. Helium nucleus or alpha particle, He 2+ or αĬalculate the expected atomic weight of the isotope of chlorine that has 20 neutrons. 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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