During the days of Newton, the atom was pictured to be a tiny, solid, indestructible sphere. This model sufficed for the discussion on the kinetic theory of gases but fell apart when the electrical nature of atoms was discovered. In 1897, Joseph John Thompson determined the charge-to-mass ratio for electrons and the following year he suggested a model of the atom which suggests that the atom as a region in which positive charge is spread out in space with electrons embedded throughout the region, much like raisins in a pudding.
However, this ‘pudding’ model of the atom lasted little more than a decade as Ernest Rutherford, together with his students Hans Geiger and Ernest Marsden, performed an experiment that would debunk Thompson’s model of the atom. A beam of positively charged alpha particles was incident onto a thin metallic foil. Most of the particles pass through the foil target as though they were passing through empty space. However, the researchers were surprised when they actually observed that many of the particles were deflected through large angles. Some of the particles were even deflected backwards!
Such observations would not be possible if the atom was as Thompson suggested since the positive charge would be spread out throughout the atom. This meant that there would not be a concentration of positive charge strong enough to cause any large-angle deflections of the incoming positively charged alpha particles. Electrons are also not responsible for these deflections as they are much less massive than the alpha particles.
Thus, in order to explain his astonishing results, Rutherford came up with a new model of the atom. The positive charge in the atom was concentrated in a small region at the centre of the atom known as a nucleus. The size of the nucleus is much smaller than the volume of the atom. The electrons belonging to the atom would then orbit the nucleus in the large empty space of the atom, just like planets around our sun.
There are two basic problems with Rutherford’s planetary model though. Firstly, this model cannot explain why an atom emits and absorbs characteristic frequencies of electromagnetic radiation. Secondly, according to the classical theory of electromagnetism, centripetally accelerated charges revolving at frequency f would emit electromagnetic waves of the same frequency. This would imply that the electrons would continually lose energy and fall towards the nucleus, much like a satellite, leading to the collapse of the atom.
These difficulties in modeling the atom eventually lead to Bohr coming up with his atomic model that improved on the erroneous model devised by Rutherford.