Discovery

Along with Hans Geiger and Ernest Marsden he carried out the Geiger–Marsden experiment in 1909, which demonstrated the nuclear nature of atoms. It was his interpretation of this experiment that led him to formulate the Rutherford model of the atom in 1911 — that a very small positively charged nucleus was orbited by electrons.
In Cambridge in 1919 he became the first person to transmute one element into another when he converted nitrogen into oxygen through the nuclear reaction 14N + α → 17O + p.
In 1921, while working with Niels Bohr (who postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keeping the nuclei from breaking apart.

Experimental Basis

From purely energetic considerations of how far alpha particles of known speed would be able to penetrate toward a central charge of 100 e, Rutherford was able to calculate that the radius of his gold central charge would need to be less than 3.4 x 10-14 meters. This was in a gold atom known to be 10-10 metres or so in radius -- a very surprising finding, as it implied a strong central charge less than 1/3000th of the diameter of the atom. The Rutherford model served to concentrate a great deal of the atom's charge and mass to a very small core, but didn't attribute any structure to the remaining electrons and remaining atomic volume. The so-called plum pudding model of J. J. Thomson had also had rings of orbiting electrons.
However, Rutherford did not attempt to make the direct connection of central charge to atomic number, since gold's place on the periodic table was known to be about 79 u, and Rutherford's more tentative model for the structure of the gold nucleus was 49 helium nuclei, which would have given it a mass of 196 u and charge of 98 e, which was much more in keeping with his experimentally-determined central charge for gold in this experiment of about 100 e. A month after Rutherford's paper appeared, the proposal regarding the exact identity of atomic number and nuclear charge was made by Antonius van den Broek, and later confirmed experimentally within two years, by Henry Moseley.

Key Points:
1) The electron cloud of the atom does not influence alpha particle scattering.
2) A large number of the atom's charges, up to a number equal to about half the atomic mass in hydrogen units, are concentrated in very small volume at the center of the atom. These are responsible for deflecting both alpha and beta particles.
3) The mass of heavy atoms such as gold is mostly concentrated in the central charge region, since calculations show it is not deflected or moved by the high speed alpha particles, which have very high momentum in comparison to electrons, but not with regard to a heavy atom as a whole.

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