According to Bohr, the fact that the light emitted by hydrogen occurs only at a unique set of frequencies implies that electrons are constrained to jump between different allowed energy levels in a particular element. The allowed energy levels are said to be quantized. The energy released when an electron jumps to a lower allowed energy level is the source of the photons that give hydrogen gas its unique emission spectrum. Bohr proposed that the orbits allowed to the electron were those defined by
in which n is the principal atomic number, a number that indicates which orbit the electron occupies. Bohr's theory predicts the distance at which the electrons orbit by the formula
in which m is the mass of the electron, e is the charge of the electron, and Z is the atomic number of the element.
