Neils Bohr proposed that;
a. Electrons could only orbit the nucleus at a fixed distance, its distance from the nucleus (i.e., their radii) being proportional to its energy.
b. Only orbits with certain energies were allowed, in other words, charge is quantized.
c. The energy of the electron in a given orbit is proportional to it's distance from the nucleus.
d. Light is absorbed when an electron moves to a higher orbit.
e. Light is emitted when an electron falls to a lower energy orbit.