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 throughNe 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