Answer:
Different isotopes for elements
Explanation:
Their stability is determined by the ratio of the number of neutrons to the number of protons in the nucleus. At low atomic masses, the stable ratio is approximately 1:1. At about an atomic mass number of 20 this starts to increase until it is around 1.5:1 for the very heavy elements. This is due to the fact that with higher numbers of protons more neutrons are needed due to the repulsion of the protons from electrostatics.
