A sulfur hexafluoride molecule, SF6, has six atoms attached to the central S atom
and no lone pairs on that atom (8). According to the VSEPR model, the electron
arrangement is octahedral, with four pairs at the corners of a square on the equator
and the remaining two pairs above and below the plane of the square (see Fig. 4.2).
An F atom is attached to each electron pair, and so the molecule is predicted to be
octahedral, with bond angles of 90 and 180. That too is confirmed experimentally.
The second rule of the VSEPR model concerns the treatment of multiple bonds:
Rule 2 There is no distinction between single and multiple bonds: a multiple
bond is treated as a single region of high electron concentration.
That is, the two electron pairs in a double bond stay together and repel other
bonds or lone pairs as a unit. The three electron pairs in a triple bond also stay
together and act like a single region of high electron concentration. For instance,
a carbon dioxide molecule, O£C£O
Å
Å
Å
Å
, has a linear structure similar to that of
BeCl2, even though both bonds are double bonds (9). One of the Lewis resonance
structures of a carbonate ion, CO3
2, is shown in (10). The two pairs of electrons
in the double bond are treated as a unit, and the resulting shape (11) is trigonal
planar. Because each bond, whether single or multiple, acts as a single unit, to
count the number of regions of high electron concentration and determine the
electron arrangement of an ion or molecule we simply count the number of atoms
attached to the central atom and add the number of lone pairs.
When there is more than one “central” atom, we consider the bonding about
each atom independently. For example, to predict the shape of an ethene (ethylene)
molecule, CH2£CH2, we consider each carbon atom separately. From the Lewis
structure (12) we note that each carbon atom has three regions of high electron
concentration (two single bonds and one double bond). The electron arrangement
around each carbon atom is therefore trigonal planar. There are two H atoms and
one C atom attached to each “central” C atom. We predict that the HCH and HCC
angles will both be close to 120 (13); this prediction is confirmed experimentally
(the actual bond angles being 117 and 122, respectively). Experimental observations
also show that all six atoms lie in a plane. This planarity can also be predicted
by the more sophisticated valence-bond theory, as we shall see shortly, and is
another instance of the refinement of a model guided by experimental evidence.