The theme of this chapter has been the special stability of a closed
inner shell of two electrons, and of a filled second shell of eight
electrons. Atoms such as lithium, which have only a few electrons
outside the inner shell, will lose them easily and show a low electronegativity.
In contrast, O and F, which lack only one or two electrons of completing
the second shell, will attract electrons and show a high electronegativity.
Electronegativity describes in relative terms how bonded atoms will
compete for electrons.
Covalent and ionic bonds are extreme types, with most actual bonds
being intermediate in character. Whether a bond is described as
mainly covalent or mainly ionic depends on whether the electron
pair of the bond is shared more or less equally between atoms, or
is pulled sharply toward one of them. When like atoms are bonded,
whether the bond is unlocalized and metallic (as in Li, as discussed
previously), or localized as an electron
pair shared between two particular atoms, depends on how tightly
the atoms normally hold electrons.