In 1913, Danish physicist Niels Bohr developed the first model of the atom that used the concepts of quantum theory to explain atomic spectra. The Bohr atom depicted electrons "in orbit" around a heavy nucleus, similar to the motion of planets around the sun. The model was based on Ernest Rutherford's 1911 demonstration that the atom consists of a central nucleus surrounded by a cloud of electrons. The electrons, Rutherford postulated, would have to be in constant motion, since the attraction of opposite charges would otherwise pull them into the nucleus. Because electrons are charged particles, they should radiate electromagnetic energy during orbits, eventually losing momentum and spiraling into the nucleus. The radiated energy would be expected to change continuously across the electromagnetic spectrum as the momentum of the moving particle changed.

This was not the observed behavior of atoms. Under normal conditions, they do not radiate energy...