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Bohr and energy levels

Even though Rutherford had proven the existence of the nucleus, scientists were unsure how electrons fitted into this new model.

In 1913, Niels Bohr revised Rutherford's model by suggesting that the electrons orbited the nucleus in different energy levels or at specific distances from the nucleus.

By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.

Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.

Bohr's 'solar system' model of the atom is the way that most people think about atoms today.

When atoms absorb energy, the electrons at a particular level are pushed up to higher levels (at bigger distances from the nucleus). In time, they jump back down to a lower level releasing light of definite frequencies.

Chadwick and the neutron

The proton had been proposed as being the nuclear particle responsible for the positive charge of the nucleus and for some of the nuclear mass. However, there was still a difference between the atomic number of the atom and the atomic mass.

Bohr and other scientists knew that there also had to be a neutral particle the same size as a proton to keep the nucleus stable and to make up the mass. This was accepted and scientists used models that included the neutron for a further 20 years before it was actually discovered. It wasn't until 1932 that James Chadwick was able to prove its existence.

Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil. He was able to prove that a proton-sized neutral particle - now known as the neutron - existed.