binding energy The mass of a nucleus is always less than the sum of masses of the neutrons, and the protons of which it is composed. The energy equivalence of the difference of mass of the nucleus, and the total mass of the particles constituting the nucleus is the binding energy of the nucleus. Obviously it is the energy that holds the nucleus together. Binding energy can be defined as the amount of energy required to break the nucleus into isolated particles.

The binding energy can be found from the mass defect D m, which is defined as,

D m = ( Z m_H + N m_n ) - m (b7)

where Z is the number of protons, N the number of neutrons, m_H the mass of hydrogen atom (consisting of a proton and an electron), m_n the mass of a neutron, and m the mass of the atom. The masses m_H, m_n in atomic mass unit (u) is given by,

m_H = 1.007825u

m_n = 1.008665u

The value of u = 931 MeV. To obtain the binding energy D m will have to be multiplied by the conversion factor 931 MeV/u.

The graph of binding energy / nucleon (BE/A), against mass number A is shown in the fig.b4. BE/A increases sharply up to mass number 50 to 60, after which it decreases slowly. This graph indicates that there are two ways by which energy can be released from the nucleus. The first is to split a large nucleus into smaller nuclei (fission). The other possibility is to combine smaller nuclei into a large nucleus (fusion).