Atoms interact with one another to form bonds in molecules by gaining,
losing, or sharing electrons. A key idea in chemistry is the great
stability of filled electron shells around an atom. Since lithium
has only one electron outside a filled helium core, it can lose
that electron easily and become a positively charged ion:
lithium atomlithium
ion
This ionization is shown schematically opposite(top). Much
more energy would be needed to remove electrons from the first or
inner shell, and this ordinarily does not happen in chemical reactions.
A beryllium atom, with two electrons outside the closed helium shell,
can lose both electrons to become a doubly charged ion, as shown
opposite(middle):