Some of the important postulates of the Bohr’s model of the atom were that the electrons moved around the nucleus in specified orbits. In these orbits they can move without radiating energy.
In 1902, Max Planck was able to show that experimental observations in the radiation emitted by substances could be explained on the basis that the energy from such bodies emitted in separate or discrete packets of energy known as energy quanta of value hf, where f is the frequency of radiation and h is a constant known as Planck’s Constant. Thus the energy E of the quantum of radiation or photon is given by E = hf.
This is known as Planck’s theory of radiation. The term quantum means amount fixed amount, or discrete or separate amount as distinguished from a continuous quantity.
Planck’s quantum hypothesis thus suggests (and this is accepted today) that the energy of radiation can be E = hf, or 2hf or 3hf, etc. but there cannot be vibrations or radiations whose energy lies between these values. That is energy radiated is not a continuous quantity but is rather quantized – i.e., it exists only in discrete amounts. This is the meaning of the concept of energy quantization.