The variable Z designates an element’s atomic number, the number of protons in its nucleus. Z is also equal to the number of electrons orbiting the nucleus in neutral (non-ionized) atoms, and is close to the number of electrons in most ions. A neutral iron atom (Fe0), for example, contains 26 protons (Z = 26) in its nucleus and 26 electrons in an electron cloud around the nucleus. Because the size of its electron cloud controls the diameter of an atom, elements with greater atomic number, with many protons and thus many electrons, are larger than those or lower atomic number.
Atomic nuclei (except one isotope of hydrogen) contain neutrons in addition to protons, and the number of neutrons, designated by N, may vary. This leads to isotopes of different mass numbers. Mass number, designated by the variable A, is equal to the number of protons and the number of neutrons combined: A = Z + N. Most chemical elements have several different naturally occurring isotopes. Some isotope varieties, however, are generally more common than others, and some only exist in minute amounts.
2.5 Isotopes of Oxygen
Oxygen atoms, for example, may be any of three isotopes (shown in Figure 2.5). Oxygen may be 16O, 17O, or 18O, where the superscript number denotes A. Examination of the equation in the previous paragraph tells us that the three isotopes of oxygen must have 8, 9, and 10 neutrons, respectively, because all must have 8 protons if they are oxygen. 17O and 18O are very rare; 16O is 99.8% of all natural oxygen.