I was taught that electrons only exchange in orbitals of equal energy or degenerate orbitals. The energy of 3d orbitals is 5 and that of 4s is 4(by n+l rule), but then how is it that in the case of copper and chromium electrons exchange from 3d orbital to 4s?
A sort of simplified answer but here goes...
Without some other excitation, electrons only "fall in" towards the nucleus. This is sort of like the fact that rocks roll down hill not up hill. So once an electron "falls in" it has excess energy that must be shed. It can either give the energy off as a photon or transfer the energy to one or more other electron(s) which then go into some excited state (Auger effect).
The transitions occur according to selection rules. If the selection rules don't allow a particular transition then it is forbidden. If the atom is in free space with no other perturbations, then forbidden transitions would not occur. However for molecules, in liquids and gases, because of vibrational and rotational microstates such a transition may actually occur but it will be relatively weak in intensity to the transitions allowed by the selection rules. Likewise forbidden transitions may occur in solids because of the vibrations of the atoms within the crystal which break the symmetry of an atom, or its excited state.
Now if an atom or molecule is excited by an external energy source, then an electron can move from a lower energy orbital to a empty higher energy orbital. Again these transitions obey the selection rules.