A model of the atom was described by the British physicist Ernest Rutherford in 1911, and is known as the Solar System model.
An atom consists of a central nucleus. This nucleus is composed of positively charged protons, and electrically uncharged (neutral) neutrons.
Negatively charged electrons revolve round the nucleus in definite orbits.
The orbits themselves can be at any distance from the nucleus.
In any atom, the number of protons is equal to the number of electrons, and hence it is electrically neutral
1. Inherent Instability of the Atom
According to Rutherford's theory, electrons could orbit the nucleus at any distance. When the electrons circle round the nucleus, they are constantly changing their direction. According to classical electrodynamics (which deals with the motion of electrons), such electrons which either constantly change their direction or their velocity or both should continuously emit radiation. While doing so, they should lose energy, and thus spiral into the nucleus. This means every atom is unstable, quite contrary to our observation
Rutherford's description of the atom could not be entirely correct because it did not account for some observations that had already been made. Perhaps the most important of these observations concerned the behaviour of certain gases. These gases at low pressure emit light in a set of discrete bands of the electromagnetic spectrum. This is quite different from the radiation emitted by solids, which is spread evenly across the electromagnetic spectrum. The radiation emissions of these gases were important because they showed that at least under some circumstances, the orbits of the electrons could not be at just any distance from the nucleus, but were confined to discrete distances (or energy states).