Planck’s Constant h and the atom
Bohr (and others) noted that the combination a0 = (h/2p)2/ me2 has the units of length about the size of atoms
Bohr postulated that it was not the atom that determined h, but h that determined the properties of atoms!
Since the electron is bound to the nucleus by electrical forces, classical physics says that the energy should be E = - (1/2) e2/a0
If the radii are restricted to certain values, the the energy can only have certain values