The number of atoms that a central atom has around
itself in a particular molecule or ion is its coordination number,
CN. For the second-shell elements Li through B, the maximum coordination
number with oxygen is four, but the smaller carbon and nitrogen
atoms have room for no more than three oxygen atoms.
In the third row, Si through Cl have a maximum CN
of four with oxygen, but the larger Na, Mg, and AI atoms at the
beginning of the row all have a maximum CN of six.
Thus a sodium ion in solution has six water molecules
around it in octahedral coordination (i.e., like the six Cl-
ions around a Na+ ion in a salt
crystal-at the four corners of a square plus one above and one below).
When amphoteric aluminum hydroxide is dissolved by
a sufficiently strong base, Al3+
is surrounded by an octahedron of six hydroxide ions to form an
ion, rather than four hydroxide ions as with beryllium, Be(OH)42-.
The difference in coordination number arises from
the relative sizes of the ions. Of the second-row elements, only
Li and Be are metals. The metal-nonmetal borderline moves diagonally
one column to the right from Row 2 to Row 3, and three of the third-row
elements are metals.