has six outer electrons, two lone pairs and two unpaired electrons
that are easily available for bonding. When oxygen makes two covalent
bonds as in water, H2O,
it is surrounded by four electron pairs, two bonding pairs and two
lone pairs, in roughly tetrahedral arrangement (see right). This restriction
to four electron pairs around the atom, which we have seen for nitrogen
and carbon, is mainly a consequence of the small size of the atoms.
An oxygen atom does not share more of its outer-shell electrons because
there is no room around it for more than four neighbor atoms. In larger
atoms with the same six electron outer-shell structure of oxygen,
such as sulfur (third shell), selenium, and tellurium, all six of
the electrons can be shared, thereby bringing six electron pairs around
the central atom. But with the small second-row elements, four pairs
is the maximum, and an eight-electron shell is full.