Bohr at the Rutherford Lab Everyone at the time imagined the atom as a "plum pudding." That is, it was roughly the same thing throughout, with negatively charged electrons scattered about in it like raisins in a pudding. As part of an experiment with x-rays in 1909, Rutherford was shooting a beam of alpha particles at a sheet of gold foil only 1/3000 of an inch thick, and tracing the particles' paths. Most of the particles went right through the foil, which would be expected if the atoms in the gold were like a plum pudding. But every now and then, a particle bounced back as though it had hit something solid. After tracing many particles and examining the patterns, Rutherford deduced that the atom must have nearly all its mass, and positive charge, in a central nucleus about 10, 000 times smaller than the atom itself. All of the negative charge was held in the electrons, which must orbit the dense nucleus like planets around the sun.
In 1912 Bohr joined Rutherford. He realized that Rutherford's model wasn't quite right. For one thing, the orbiting electrons should give off energy and eventually spiral down into the nucleus, making the atom collapse. Or the electrons could be knocked out of position if a charged particle passed by. Bohr turned to Planck's quantum theory to explain the stability of most atoms.
He found that the ratio of energy in electrons and the frequency of their orbits around the nucleus were equal to Planck's constant. Bohr suggested the idea that electrons "jump" between energy levels in a quantum fashion. But when an atom absorbs or gives off energy (as in light or heat), the electron jumps to higher or lower orbits. Bohr published these ideas in 1913 to mixed reaction. But there was good evidence he was right: the electrons in his model lined up with the regular patterns of light emitted by real hydrogen atoms. The shells in which electrons orbit hav different quantum numbers and hold only certain numbers of electrons - the first shell holds no more than 2, the second shell up to 8, the third 10, the fourth 14.
Atoms with less than the maximum number in their outer shells are less stable than those with "full" outer shells.