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The isotopes of an element are atoms of the element having the same atomic number but different atomic masses – the phenomenon is known as isotopy. The difference in their atomic masses is due to differences in their number of neutrons.

Having same number of protons, the isotopes of an element, except those of hydrogen all exhibit similar chemical properties.

The expressed atomic masses are actually the average mass of all the isotopes. Many elements exhibit isotopy. A very common one is chlorine – 35Cl and 37Cl.

Atoms of 35Cl are most abundant, consisting of about 75% of the total isotopes, while the abundance of the 37Cl atoms is about 25%. The average atomic mass of an element exhibiting isotopy can be deduced by taking the sum of the products of relative abundance and the masses of the different isotopes. I.e., the average atomic mass of chlorine is:

75/100 x 35 + 25/100 x 37 = 26.25 + 9.25 = 35.5 where 75/100 is the relative abundance of the isotope of mass 35 and 25/100 is the relative abundance of the isotope of mass 37.

Note: the discovery of isotopy has faulted the idea in the Dalton’s atomic theory which states that all atoms of the same element are similar in every respect. Because of the presence of isotopes in some elements, their relative atomic masses will not be whole numbers, since they are not present in the same proportions.

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