2 Bohr's atomic model (1913)

• The Danish scientist Niels Bohr explained the hydrogen spectrum in 1913 (Nobel Prize in Physics, 1922).
• He kept three ideas of Rutherford: (1) a positive nucleus at the centre; (2) the number of electrons equals the number of positive charges in the nucleus; (3) a centrifugal force balances the attraction of the nucleus.
• Then he added the following postulates:

4. Electrons move rapidly around the nucleus without losing or gaining energy while they stay in a given orbit.
5. Electrons move in a number of definite energy levels; the spaces between them are forbidden regions for electron motion.
6. An electron has a definite energy depending on the distance of its level from the nucleus; the energy increases as the radius of the level increases. Each level is labelled by an integer, the principal quantum number n (1, 2, 3, …).
7. Normally the electron stays in the lowest possible energy level (ground state, stable). If it gains a quantum of energy (by heating or electric discharge) the atom becomes excited and the electron jumps to a higher level corresponding to the quantum absorbed. This state is unstable; the electron soon returns, losing the same quantum as light of a characteristic wavelength and frequency — producing a spectral line.
8. Many atoms absorb different quanta at the same time and many emit others, so a set of lines appears, indicating the energy levels between which the electrons jump. This explains the hydrogen lines.

A quantum is the amount of energy gained or emitted when an electron moves from one energy level to another.