The theory of the structure of the atom has changed drastically over the course of its development. In 1913, Neils Bohr proposed an explanation for the existence of each atom's unique spectral emission lines. When hydrogen gas is subjected to a high voltage, it emits light at specific frequencies. The frequencies of emission are unique for each element.
Bohr suggested that electrons do not obey the laws of classical mechanics in the traditional style. If they did, the electrons orbiting the nucleus would gradually lose potential energy and gain kinetic energy as they spiraled in toward the nucleus. Einstein had already shown (over ten years earlier) that the frequency of light emitted by a particle (v) equals the radiant energy of the light (E) divided by Planck's constant (h). This relationship is generally expressed in the form
