Bohr at the Rutherford Lab

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 …

…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 have 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.