The binding energy of a nucleus, is the work done or energy needed to separate completely the nucleons (protons and neutrons) apart. Again, experiment shows that the total mass of all individual nucleons is greater than that of the nucleus, in which they are together. This difference in the mass is the measure of the binding energy of the particular nucleus.

For helium	4	He,
	2

it has altogether 4 nucleons 2 protons and 2 neutrons
2:H = 2 x 1.00783 = 2.01566
2:n = 2 x 1.00867 = 2.01734
∴ 2:H + 2:n = 4.03300u
But the helium nucleus has a mass of 4.00260u
∴ Binding energy = difference of nucleons and nucleus
= 4.03300U - 4.00260U = 0.03040U