Thomson's–Plum pudding model.
Before Ernest Rutherford's landmark experiment with a few pieces of metal foil and alpha particles, the structure of the atom was thought to correspond with the plum pudding model. In summary, the plum pudding model was hypothesized by J.J. Thomson (the discoverer of the electron) who described an atom as being a large positively charged body that contained small, free–floating, negatively charged particles called electrons. The plum pudding model also states that the negative charge of the electrons is equivalent to the positive charge of the rest of the atom. The two charges cancel each other and cause the electrical charge of the atom to be zero (or neutral). The faulty aspect of this model is that it was constructed before the nucleus of an atom (and it's composition) was discovered, which is where Rutherford's research comes in.
Sir Ernest Rutherford (1871–1937)
New Zealand physicist, Rutherford contributed to the understanding of atomic structure. In 1909, he fired
helium ions (alpha particles) at thin gold foil and observed the scattering pattern. A few particles were deflected by large
amounts, implying that the atomic nucleus is very small and positively charged. He won the 1908 Nobel Prize for chemistry for
showing that radioactivity was the spontaneous decay of atoms, and that the time taken for half of a radioactive sample to decay
was constant (the half life).
In 1911, Ernest Rutherford conducted an experiment that proved that the mass of an atom is concentrated in the center (nucleus) of an atom. It also proved that an atom is mostly empty space.
When he shot a beam of alpha particles at a sheet of gold foil, a few of the particles were deflected. He concluded that a tiny, dense nucleus was causing the deflections.
Rutherford carried out a series of experiments using very thin foils of gold and other metals as targets for a particles from a radioactive source. They observed that the majority of particles penetrated the foil either undeflected or with only a slight deflection. But every now and then a particle was scattered (or deflected) at a large angle. In some instances, a particle actually bounced back in the direction from which it had come! This was a most surprising finding, in Thomson's model the positive charge of the atom was so diffuse that the positive a particles should have passed through the foil with very little deflection.
Rutherford was later able to explain the results of the α-scattering experiment in terms of a new model for the atom. According to Rutherford, most of the atom must be empty space. This explains why the majority of a particles passed through the gold foil with little or no deflection. The atom's positive charges, Rutherford proposed, are all concentrated in the nucleus, which is a dense central core within the atom. Whenever a particle came close to a nucleus in the scattering experiment, it experienced a large repulsive force and therefore a large deflection. Moreover, a particle traveling directly towards a nucleus would be completely repelled and its direction would be reversed.
Rutherford's Gold Foil Experiment
Rutherford Explanations on gold foil experiment :
- Since most of the alpha particles pass straight through the gold foil without any deflection, it shows there is a lot of empty space in an atom.
- Those positively charged alpha particles deflected by large angles–some even backward, nearly in the direction from which they had come, which shows that there is a positive charge in center which is not distributed uniformly inside the atom.
Rutherford's Gold Foil Experiment showing deflections
- Around 1 in 8000 alpha particles were deflected by very large angles (over 90°), while the rest passed straight through with little or no deflection. From this, Rutherford concluded that the majority of the mass was concentrated in a central core.
- An atom has a tiny positively charged core (nucleus) which contains most of the mass over 99.9% of the mass of the atom.
- The electrons revolve around the nucleus in circular paths similar to the planets revolve around the Sun (solar system). The electrostatic force of attraction between the nucleus and electron provides centripetal force.
- He estimated that the radius of the nucleus was at least 1/100000 times smaller than that of the radius of the atom. Scientists imagined the size of the nucleus with the following similarity, if the size of the atom is that of Earth then the nucleus would have the size of an apple.
- The amount of positive charge in the nucleus is equal to the amount of negative charge on the electrons. So, the atom as a whole is electrically neutral.
One of the most important limitation of Rutherford model is that Rutherford's model failed to explain stability of atoms or why electrons which revolve around the nucleus do not lose energy and finally fall into the nucleus. Stability of atoms is explained by Bohr model of atom.