Although most of the lighter elements have atomic masses that are nearly whole numbers, some elements were discovered to have atomic masses that could not be integral. In Figure 1, look at the atomic masses of the three lightest halogens and satisfy yourself that although the values for fluorine and bromine may be whole numbers, the value of chlorine is definitely intermediate. The atomic mass of chlorine is not close to a whole number.

Figure 1. The three lightest halogens.

The interpretation of the curious mass of chlorine awaited the discovery of the neutron by James Chadwick in 1932. Although all chlorine atoms have 17 protons, different isotopes of the element have different numbers of neutrons. In Table 1, the mass numbers of the chlorine isotopes are denoted by superscripts to the upper left of the chemical symbol.

The nonintegral atomic mass for naturally occurring chlorine is seen to be the weighted average of the atomic mass of its two major isotopes found by multiplying the atomic mass of each isotope times its decimal equivalent of its relative abundance:

Now perform that calculation in the opposite direction. Beginning with the known atomic mass of natural chlorine, determine the abundance of the two isotopes:

Instead of using the integers 37 and 35 as atomic masses, take the more precise atomic masses of the isotopes from Table 1:

The calculation reveals that natural chlorine is 24%³⁷Cl and 76%³⁵Cl.

The most carefully studied element is the simplest, hydrogen, which has a natural atomic mass (1.0080) slightly greater than that of a single proton (1.0078). (See Table 2.) This mass excess is only 0.0002 atomic mass units, but the investigation of this excess revealed the three isotopes of that element.

²H is often called deuterium, and³H is referred to as tritium. The atomic mass of natural hydrogen (1.0080) exceeds that of¹H because of the admixture of deuterium:

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