http://en.wikipedia.org/wiki/Nuclear_binding_energy What is nuclear binding energy ? Here are two possible answers according to the Wikipedia page : 1.Nuclear binding energy is the energy required to split a nucleus of an atom into its component parts. 2.Nuclei are made up of protons and neutron, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together. Which one is it ? Or are both of them trying to tell the same thing ?