<P> Thomson's plum pudding model was disproved in 1909 by one of his former students, Ernest Rutherford, who discovered that most of the mass and positive charge of an atom is concentrated in a very small fraction of its volume, which he assumed to be at the very center . </P> <P> In the Geiger--Marsden experiment, Hans Geiger and Ernest Marsden (colleagues of Rutherford working at his behest) shot alpha particles at thin sheets of metal and measured their deflection through the use of a fluorescent screen . Given the very small mass of the electrons, the high momentum of the alpha particles, and the low concentration of the positive charge of the plum pudding model, the experimenters expected all the alpha particles to pass through the metal foil without significant deflection . To their astonishment, a small fraction of the alpha particles experienced heavy deflection . Rutherford concluded that the positive charge of the atom must be concentrated in a very tiny volume to produce an electric field sufficiently intense to deflect the alpha particles so strongly . </P> <P> This led Rutherford to propose a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge . Only such a concentration of charge could produce the electric field strong enough to cause the heavy deflection . </P> <P> The planetary model of the atom had two significant shortcomings . The first is that, unlike planets orbiting a sun, electrons are charged particles . An accelerating electric charge is known to emit electromagnetic waves according to the Larmor formula in classical electromagnetism . An orbiting charge should steadily lose energy and spiral toward the nucleus, colliding with it in a small fraction of a second . The second problem was that the planetary model could not explain the highly peaked emission and absorption spectra of atoms that were observed . </P>

When was the first theory of atoms proposed and by who