Why is fusion only possible with small nuclei?

When atoms undergo fusion, energy is released. This is due
to an increase in the binding energy between the particles that make up the nucleus.
Binding energy is a negative energy; in the sense that it requires an investment of
energy to split the constituent particles of a nucleus.

The
binding energy arises from a nuclear force. This acts between 2 neutrons, 2 protons and
between a neutron and a proton. The nuclear force keeps the nucleus from breaking apart
due to the repulsive forces between the positively charged protons. The measurement of
the mass of nuclei has revealed that the mass of the constituent protons and neutrons is
more than the mass of the nucleus. The decrease in mass is converted to energy which is
given off.

Two atoms undergo fusion if the binding energy
of the resultant atom is more than that of the atoms which fuse. This is the case with
atoms of light elements undergo fusion, for example helium has a higher binding energy
than hydrogen. It is seen that the difference in the binding energy continues to
decrease as the atoms undergoing fusion become heavier. Iron has the highest binding
energy. Beyond iron there is no increase in the binding energy, this makes fusion
reactions come to a stop at iron.

After iron, energy is
released when the atoms are split into smaller atoms. This is the principle behind the
generation of energy by nuclear fission. For example uranium atoms release energy not
when they combine to form a larger atom but the other way
around.