In 1906 Rutherford began to study what happened when massive subatomic particles, such as alpha particles, passed through matter. When alpha particles passed through a thin film of gold, for instance, they raced through, for the most part, as though nothing were there. The alpha particles seemed to push the light electrons aside and to act as though the positively charged main body of the atom that Thomson had pictured was not solid, but was soft and spongy. The only trouble was that every once in a while an alpha particle seemed to strike something in the gold film and bounce to one side. Sometimes it even bounced directly backward. It was as though somewhere in each atom there was something at least as massive as the alpha particle. How large was this massive portion of the atom? It couldn’t be very large for if it were the alpha particles would hit it frequently. Instead, the alpha particles made very few hits. This meant the massive portion was very small and that most alpha particles tore through the atom without coming anywhere near it. [Illustration: _Rutherford’s alpha particle bombardment apparatus. A piece of radium in the lead box (B) emits alpha particles that go through the gold foil (F). These particles are scattered at different angles onto the fluorescent screen (S), where the flashes caused by each impact are seen through the microscope (M). Below, alpha particles are shown bouncing off a nucleus in the gold foil._] [Illustration: ] By 1911 Rutherford announced his results to the world. He suggested that just about all the mass of the atom was concentrated into a very tiny, positively charged “nucleus” at its center. The diameter of the nucleus was only about 1/10,000 the diameter of the atom. All the rest of the atom was filled with the very light electrons. [Illustration: _Hans Geiger (left) and Ernest Rutherford at Manchester University about 1910._] According to Rutherford’s notion, the atom consisted of a single tiny positively charged lead shot at the center of a foam of electrons. It was Thomson’s notion in reverse. Still, the nucleus carried a positive charge of a particular size and was balanced by negatively charged electrons. Rutherford’s model of the atom explained the existence of ions just as easily as Thomson’s did and it explained more besides. For instance, if all the electrons are removed so that only the nucleus remains, this nucleus is as massive as an atom but is so tiny in size that it can penetrate matter. The alpha particle would be a bare atomic nucleus from this point of view. Rutherford’s model of the “nuclear atom” is still accepted today. Atomic Numbers Since the atom consisted of a positively charged nucleus at the center, and a number of negatively charged electrons outside, the next step was to find the exact size of the nuclear charge and the exact number of electrons for the different varieties of atoms.