They can therefore only absorb energy in quantities which correspond to differences between two of these allowed energy levels. The energy of a photon is proportional to its frequency according to the Planck relation and the wavelength and frequency are related by . Thus, only wavelengths of light whose corresponding photon energies coincide with a difference between two energy levels of hydrogen can be absorbed. In all of the following, the Lewis diagram is on the left. A diagram of the shape is shown on the right.
Ammonium is tetrahedral.
The Lewis diagram of below also shows its bent shape.