Example: CH 4 , methane
TIP
Configurations often appear as questions on the SAT test.- Mutual repulsion of six identical electron clouds directs them to the
corners of an inscribed regular octahedron. This is said to have an
octahedral arrangement.
Example: SF 6 , sulfur hexafluoride
VSEPR and Unshared Electron Pairs
Ammonia, NH 3 , and water, H 2 O, are examples of molecules in which the central
atom has both shared and unshared electron pairs. Here is how the VSEPR theory
accounts for the geometries of these molecules.
The Lewis structure of ammonia shows that, in addition to the three electron
pairs the central nitrogen atom shares with the three hydrogen atoms, it also has
one unshared pair of electrons:
VSEPR theory postulates that the lone pair occupies space around the
nitrogen atom just as the bonding pairs do. Thus, as in the methane molecule
shown in the preceding section, the electron pairs maximize their separation by
assuming the four corners of a tetrahedron. Lone pairs do occupy space, but our
description of the observed shape of a molecule refers to the positions of atoms
only. Consequently, as shown in the drawing below, the molecular geometry of an
ammonia molecule is that of a pyramid with a triangular base. The general
VSEPR formula for a molecule such as ammonia (NH 3 ) is AB 3 E, where A
replaces N, B replaces H, and E represents the unshared electron pair.
A water molecule has two unshared electron pairs and can be represented as