This was a real problem to the physicists at
the beginning of the twentieth century. Since the electrons are negative and the nucleus was
positive there seemed no reason why the orbiting electrons should not simply spiral down to
meet the nucleus, emitting a puff of radiation as they did so. However if this were the case
the universe, as we know it, should have disappeared millions of years ago.
In 1913
Neils Bohr used the ideas of quantum physics to say why this did not happen. He simply
proposed that there were only certain allowed orbits around the nucleus and that there was a
lower orbit from which they could not fall. In other words the energy of the electron was
quantised – it could only have certain values.
These orbits are all described by the
energy of the electrons within them and are called energy levels. Only certain energy levels
are allowed and as the electrons move from one level to another they either absorb energy
(to move them to a higher level) or emit energy as they fall to a lower energy level. The
lowest energy level or ground sate is the smallest amount of energy that the electrons in a
given atom can have and they cannot 'fall' closer to the nucleus than that.