Atoms of the same element all contain the same number of protons, but the number of neutrons may be different. Forms of an element which may differ in the number of neutrons are called isotopes. For example, the element chlorine with relative atomic mass 35.5, consists of two kinds of atom with different mass numbers.
Figure 1 THE ISOTOPES OF CHLORINE
Since the chemical reactions of an atom depend on its electrons, all chlorine atoms react in the same way. The number of neutrons in the nucleus does not affect chemical reactions. The different forms of chlorine are isotopes. Their chemical reactions are the same. In any sample of chlorine, there are three chlorine atoms with mass 35 u for each chlorine atom with mass 37 u so the average atomic mass is
That is why the relative atomic mass of chlorine is 35.5 u
Isotopes are shown as:
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