Binding Energy
As the protons and neutrons come together the strong nuclear force pulls them closer and they lose potential energy. (Like how an object loses its gravitational potential energy as it falls to the Earth.)
Energy must be done against the s.n.f. to separate the nucleus into the nucleons it is made of. This is called the binding energy (although ‘_un_binding’ energy would be a better way to think of it).
The binding energy of the Helium nucleus from above would be: E = m c2 E = (0.048 x 10-27) x (3.0 x 108)2
E = 4.32 x 10-12 J
The Joule is too big a unit to use at the atomic scale. We will use the electron Volt (see AS Unit 1)
1u = 1.5 x 10-10 J and 1eV = 1.60 x 10-19 J 1u = 931.3 MeV
We can now calculate the binding energy of the Helium nucleus to be: E = 27 MeV (27 million eV)