More energy is required to remove this electron because a negative charge is being removed from an object that is already positively charged. Even more importantly, this electron comes from the inner electron shell, which is not shielded by an intervening pair of electrons and therefore feels the full +3 attraction of the nucleus. The even larger third IE,



arises because one now must pull a negative charge away from a highly charged +3 nucleus. There can be no fourth IE for lithium, because it only has three electrons.

If you looked at the four successive ionization energies of the beryllium atom, you would find that the first two are relatively small, but the third and fourth, representing removal of inner-shell electrons, are far higher. In general, much more energy is needed to remove electrons from a closed inner shell than to take electrons away from a partly filled outer electron shell.

First ionization energies are a good index of how tightly different atoms hold their electrons in the outermost shell. Values for the elements H through Ne are shown in the graph on the next page. First IE's for H and He are especially high because the electrons are close to the nucleus in the inner shell, and the value for He is double that of H because the charge on the nucleus is doubled.