Radioactive decay

Stable nuclei

An atom's nucleus can only be stable if it has a certain number of neutrons for the number of protons it has.

Elements with fewer protons, such as the ones near the top of the periodic table, are stable if they have the same number of neutrons and protons, for example carbon, carbon-12 is stable and has six protons and six neutrons.

However as the number of protons increases, more neutrons are needed to keep the nucleus stable, for example lead, lead-206 has 82 protons and has 124 neutrons.

Graph showing the rate of decay with he number of protons against the number of neutrons. Purple line on the graph shows the stability line and the green line is N=Z.

Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay, in a random process, emitting radiation.