It also means that the 3d orbitals are slightly closer to the nucleus than the 4s - and so offer some screening. You will find this commented on in the page about electronic structures of ions . Compare the sizes of sodium and chloride ions with the sizes of sodium and chlorine atoms. Positive ions Positive ions are smaller than the atoms they come from. You've lost a whole layer of electrons, and the remaining 10 electrons are being pulled in by the full force of 11 protons.
Negative ions are bigger than the atoms they come from.
Although the electrons are still all in the 3-level, the extra repulsion produced by the incoming electron causes the atom to expand. There are still only 17 protons, but they are now having to hold 18 electrons. To the atomic properties menu . . . To the atomic structure and bonding menu . To Main Menu .