quantum physics?

Question

Why do electrons in an atom absorb energy in quanta of certain definite amounts only?

Answer 1

The first answer is mostly correct. The only energy change happening is an increase in the kinetic energy of a particular electron, causing it to assume a different orbit with a higher energy level. The constraint is that the length of the electron orbit must be an integral multiple of the wavelength of the electron's wavefunction. That's the quantum constraint.

Answer 2

It has to do with the fact that electrons can only exist in discrete energy levels within an atom. The only way an electron can absorb energy is to have its potential energy (energy level) increased. So the electron will jump up an energy level when its hit with a photon. That photon must have an energy equal to the energy level difference.