Albert Einstein's signature

In 1905, in his theory of Special Relativity, Einstein proposed that mass and energy were indeed equivalent. They are linked by the immortal formula E = mc², where E = energy, m = mass, and c = the speed of light (300,000 kms^-1). Thus, a certain quantity of energy is equivalent to a certain mass. However, even a small amount of mass is equivalent to a massive quantity of energy.

So, one can see that the mass of the nucleus, and the energy required to bind it are just different types of the same thing. For each atom, the mass and binding energy are inversely proportional; that is, if you increase one, the other must be decreased to minimise energy expenditure.

Thus, there are two ways of splitting the atom (scission): either by increasing its energy, or by increasing its mass.

However, the energy needed to split an atom is enormous, and this is not a practical possibility. The usual result when a nucleus becomes excited (extra energy added) is that gamma or beta rays are released to reduce energy. In other cases where the nucleus is unstable, alpha or beta particles are released to reduce mass, rather than the atom splitting. An alpha particle is a helium nucleus (⁴He), and a beta particle is a single electron. By releasing mass, the atom is able to reduce energy, and become stable again.

The other method (adding mass) is achieved by bombarding a suitable nucleus with neutrons. Suitable nuclei are those with fissionability parameters close to 1 (e.g. ²³⁵U = 0.8). By adding an extra neutron to the nucleus, mass is increased, and binding energy is consequently reduced to conserve energy. When the binding energy is reduced, the electrostatic repulsion within the nucleus is greater than the binding energy, and the nucleus splits apart.