Bohr's postulates (hydrogen atom)

At the centre of the atom there is a positively charged nucleus.

Negatively charged electrons move around the nucleus in shells

• (loosely called orbits).
• Each shell has a definite energy.
• While an electron stays in one shell it does not radiate energy (these are the "stationary states").

The atom is electrically neutral: the number of electrons round the nucleus equals the number of positive charges (protons) in the nucleus.

Emission and absorption: if an electron jumps from an outer shell of higher energy E₂ to an inner shell of lower energy E₁ (E₂ > E₁), the energy difference is emitted as one photon:

h ν = E₂ − E₁

• Conversely, an electron can jump from a lower to a higher level by absorbing a photon whose energy is exactly equal to the difference of the two levels.
• A photon with a different energy is not absorbed.

Inside the atom both electric (Coulomb) forces and mechanical (Newton) laws operate: the electrostatic attraction supplies the centripetal force of the electron's circular motion.

Allowed shells (quantum condition):

• the shell radius is found by assuming that the wave accompanying the electron forms a standing wave round the orbit.
• This happens only if the circumference of the orbit contains a whole number of wavelengths:

n λ = 2π r_n, where n = 1, 2, 3, ... and λ = h/(m v) is the de Broglie wavelength.

Therefore m v r_n = n h/2π, and only particular radii are possible.