Question

Why does in potassium the M shell is completely not filled ?

Answer 1

Answer:

The 3rd shell of any atom can actually contain 18 electrons, but potassium (K) normally has only one electron in the 4s orbital. To put an electron into a 3d orbital you need to reach a -2 charge, but it's far more energetically stable for potassium to lose an electron than to gain 2.