Isotopes

Isotopes

The number of neutrons in an atom can vary.

For example an atom of chlorine can have either 18 or 20 neutrons.

The symbols for these atoms are:

They are both chlorine atoms since they both have the same number of protons, they are called isotopes.

Isotopes are atoms with the same atomic number, but different mass numbers.

Most elements are made up of a mixture of isotopes which is why their atomic masses (seen on many Periodic Tables) are not whole numbers.

Relative Atomic Mass

The relative atomic mass (RAM) is the average mass of the atoms of an element. The RAM of an element takes into account the proportions of each isotope present.

Taking chlorine as an example:

Chlorine has a relative atomic mass of 35.5.

Chlorine exists as two isotopes: chlorine-35 and chlorine-37. (Mass number are 35 and 37).

In a naturally occurring sample of chlorine, we find that 75 per cent is chlorine-35 atoms and the other 25 per cent is chlorine-37 atoms.

The RAM of an element is usually closest to the mass of the isotope with the higher proportion. For example RAM of Cl = 35.5.

Next section: Some important groups in the Periodic Table