In 1909, Rutherford's X-ray experiments shattered conventional wisdom when he discovered that electrons didn't occupy matter like evenly-distributed raisins in a pudding. However, Bohr found Rutherford's conclusions to be inaccurate and proposed the revolutionary idea that atoms behaved according to the laws of quantum physics.
Rutherford shot alpha rays, or particles, from radioactive radium at a very thin sheet of gold foil. After examining the path patterns of the few that bounced back, Rutherford concluded that the 10,000-times-smaller nucleus was positively charged and had nearly all the mass. All the negative charge was held by the orbiting electrons.
In 1912, Bohr joined Rutherford, but found his model very unstable according to conventional physics and proposed that Planck's quantum theory explained why atoms were stable. He discovered that the ratio of energy in electrons and the frequency of their orbits equaled Planck's constant of light's energy being in proportion to its wave frequency.
Bohr proposed that electrons jumped orbits, or energy levels, without existing in between. Atoms absorbed or released energy when electrons jumped to higher or lower orbits. Mixed reactions followed Bohr’s 1913 publication, but Bohr’s model showed that electrons lined up in regular patterns when light was emitted by real hydrogen atoms.
Bohr's theory shows that a certain number of electrons in certain orbits have quantum numbers. This is the key to the periodic repetition of properties of the elements. The first shell holds up to 2, the second 8, the third 10, and the fourth 14. Atoms with less-than-maximum electrons in their outer shells are less stable. Elements with equal numbers of electrons in their outermost shells are in the same column in the periodic table of elements and tend to have similar chemical properties.