The following, should be remembered in the energy level diagram-
i) Matter containing atoms and molecules, may undergo, electronic transitions, vibrations, rotations etc. Each of these, activity has energy levels for excitation. Accordingly, each of the excitation may absorb energy of different magnitudes. Depending upon the energy available in the energy source, matter may absorb energy and use it for exciting one or more of electronic, vibration, or rotation state etc., to a higher energy state(level).
ii) Energy levels are quantized. Quantization of energy levels, prevents matter not to absorb not to absorb all energies falling on it, but a selective energy only.
iii) The quantized energy gap is characteristic of each atom or molecule. So, the energy of absorption or emission is characteristic of the atom/molecule a radiation. In other words, a particular energy absorbed or emitted by one atom may not be absorbed or emitted by an another atom/molecule.
iv) Multiple excited energy levels (states) are possible for each of activity.
v) Energy of the higher levels are also quantized. Energy gap between successive energy levels decreases, such that at very high energies they tend to merge.
vi) Number of excitation between the different energy levels is limited by ‘forbidden rule’.