Question

what is quantisation of energy?

Answer 1

Quantization means that a quantity cannot vary continuously to have any arbitrary values but can change only discontinuously to have some specific values.

Now, according to the Bohr's model, an electron in an atom can be present only in those orbits in which angular momentum of the electron is whole number multiple of h/2Ï€. Moreover, these orbits are associated with the definite amount of energy.

Hence the electron in an atom can have only definite values of energy that is energy of the electron in an atom is quantised.