Neutron and proton numbers

The variation in proton and neutron numbers is a very important factor in our understanding of the behaviour of a nucleus.


The graph shows the variation of neutron number with proton number. For light atoms (such as helium) these two numbers are equal – there are as many neutrons in the nucleus as there are protons. However for more massive elements the neutron number is much greater than the proton number. For example a U235 nucleus contains 92 protons but 143 neutrons. Particles below the line (such as B) will decay by beta plus (positron) emission while those above (such as A) will decay by beta minus emission.