Isotopes

We have previously seen (in this post) that an element always has a fixed atomic number but its atoms can have different atomic mass values. This is because most elements can exist in different forms with different numbers of neutrons – but always the same number of protons and electrons. These different forms are known as isotopes.

Carbon, for example, always has six protons and therefore sits in position six of the Periodic Table (counting from left to right and top to bottom). Most carbon atoms have six neutrons, giving an atomic mass of 12, but a small amount of carbon exists with eight neutrons in the nucleus, giving an atomic mass of 14. These two atomic variants are called isotopes because they are different versions of the same thing (carbon, in this case). They are known as carbon-12 and carbon-14 respectively.

Carbon-12 is stable (it does not undergo radioactive decay) but carbon-14 is radioactive and can be used in radio-carbon dating to determine the ages of ancient objects. In general, isotopes that contain more neutrons are more likely to be unstable and are therefore found in smaller quantities in nature.

It is useful to realise that neutrons are the particles that determine the stability of an atom in nature and they are also the particles that are used to create nuclear fission artificially for generating electricity in nuclear power stations.