Isotopes

Although most of the lighter elements have atomic weights that are nearly whole numbers, some elements were discovered to have atomic weights that could not be integral. In Figure 1 , look at the atomic weights 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 weight of chlorine is not close to a whole number.

Figure 1 The three lightest halogens.

The interpretation of the curious weight of chlorine awaited the discovery of the neutron 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 weight for naturally occurring chlorine is seen to be the weighted average of the atomic weight of its two major isotopes:

Now perform that calculation in the opposite direction. Beginning with the known atomic weight 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 weights 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 3 isotopes of that element.

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

•      Problem 1: In the following chart, which nuclei are isotopes of one chemical element? Can you give the element's name? Which nuclei have nearly the same mass?

Nucleus Protons Neutrons A 11 13 B 12 12 C 12 13

•      Problem 2: Listed in the following chart are the atomic weights (measured in atomic mass units) for natural silver and its two isotopes. Use this data to calculate the percentage of silver-109 in the natural mixture.

107Ag 106.905 109Ag 108.905 natural Ag 107.868

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