the Bohr model
/ BOR /
The Bohr model is an early picture of the atom, proposed by Niels Bohr in 1913, in which electrons circle the nucleus only in certain allowed orbits, like planets locked to specific tracks. Everyday image: think of rungs on a ladder rather than a smooth ramp; the electron can stand on one rung or another but never in between. This bold picture answered a burning question of its day: why don't the orbiting electrons spiral into the nucleus, and why do atoms give off light only at sharply defined colours?
Precisely, Bohr made two radical rules. First, an electron can occupy only special orbits in which its angular momentum is a whole-number multiple of h-bar (angular momentum = n times h-bar, with n = 1, 2, 3, ...); in these orbits it does not radiate. Second, the atom emits or absorbs light only when an electron jumps between orbits, and the photon's energy exactly equals the energy gap: E_photon = h f = E_high minus E_low. For hydrogen this gives allowed energy levels E_n = -13.6 eV / n^2, and those levels predict the observed spectral lines with beautiful accuracy.
Why it matters: the Bohr model was the first to explain atomic spectra from first principles and it introduced the crucial idea of quantized energy levels, which survives in modern physics. Honest caveat: the model is not correct as a literal picture. Electrons are not tiny balls on fixed circular tracks; the true quantum atom has fuzzy probability clouds (orbitals) and no well-defined path, as required by the uncertainty principle. The Bohr model works well only for hydrogen and one-electron ions, and fails for atoms with many electrons. It is a stepping-stone, not the final truth.
When a hydrogen electron falls from the n = 3 level to the n = 2 level, it emits a photon of energy 13.6 eV times (1/4 minus 1/9), about 1.89 eV, which is red light at 656 nanometres, the famous red line seen in hydrogen lamps and in the Sun.
An electron jump emits a photon of exactly the gap energy.
The neat orbits are a useful fiction; real electrons occupy fuzzy probability clouds, and the model works well only for hydrogen-like, one-electron atoms.