To avoid confusion, the average mass of the isotopes that make up a particular element is still called the atomic weight of that element. The integer closest to the mass of the individual isotope is spoken of as the “mass number” of that isotope. Thus, chlorine is made up of isotopes with mass numbers 35 and 37, but the atomic weight of chlorine as it is found in nature is 35.5 (or, to be more accurate, 35.453).
In the same way, ordinary lead is made up of isotopes with mass numbers 204, 206, 207, and 208, and its atomic weight is 207.19; neon is made up of isotopes with mass numbers 20, 21, and 22, and its atomic weight is 20.183, and so on.
If the atomic weight of some element happens to be very close to a whole number to begin with, it may consist of a single kind of atom. For instance, the gas fluorine (chemical symbol F) has an atomic weight of nearly 19, while that of the metal sodium (chemical symbol Na) is nearly 23. As it turns out, all the atoms of fluorine are of the single variety ¹⁹F, while all the atoms of sodium are ²³Na.
Sometimes the atomic weight of an element, as it occurs in nature, is nearly a whole number and yet it is made up of more than 1 isotope. In that case, one of the isotopes makes up very nearly all of it, while the others are present in such minor quantities that the average is hardly affected.
Helium, for instance (atomic symbol He) has an atomic weight of just about 4 and, indeed, almost all the atoms making it up are ⁴He. However, 0.0001% of the atoms, or one out of a million, are ³He. Again, 99.6% of all the nitrogen atoms (atomic symbol N) are ¹⁴N, but 0.4% are ¹⁵N. Then, 98.9% of all carbon atoms (atomic symbol C) are ¹²C, but 1.1% are ¹³C. It is not surprising that the atomic weights of nitrogen and carbon are just about 14 and 12, respectively.
Harold Urey
Even hydrogen does not escape. Its atomic weight is just about 1 and most of its atoms are ¹H. The American chemist Harold Clayton Urey (1893- ) detected the existence of a more massive isotope, ²H. This isotope has almost twice the mass of the lighter one. No other isotopes of a particular atom differ in mass by so large a factor. For that reason ²H and ¹H differ in ordinary chemical properties more than isotopes usually do and Urey therefore gave ²H the special name of “deuterium” from a Greek word meaning “second”.
W. F. Giauque