and so there are eight valence electrons. The next step is to arrange the dots representing
the electrons so that the C atom has an octet and each H atom has a duplet.
We draw the arrangement shown on the left in (4); the Lewis structure of methane
is then drawn as shown on the right in (4). Because the carbon atom is linked by
four bonds to other atoms, carbon is said to be tetravalent: it has a valence of 4.
Once again, note that a Lewis structure does not in general show the shape of the
molecule, just the pattern of bonds, its “connectivity.” In Chapter 4, we shall see
that the three-dimensional arrangement of bonds in a methane molecule is in fact
tetrahedral.
The general procedure for constructing the Lewis structure of any molecule or
ion is set out in TOOLBOX 3.1 at the end of this section, but the following information
is essential for applying those rules.
A single shared pair of electrons is called a single bond. Two electron pairs
shared between two atoms constitute a double bond, and three shared electron
pairs constitute a triple bond. A double bond, such as C a aO, is written C£O in a
Lewis structure. Similarly, a triple bond, such as C a a a C, is written CΩC. Double
and triple bonds are collectively called multiple bonds. The bond order is the number
of bonds that link a specific pair of atoms. The bond order in H2 is 1; in the
group C£O, it is 2; and for CΩC in a molecule such as ethyne, C2H2, the bond
order is 3.