9.2. Molecular Geometry http://www.ck12.org
Unlike the trigonal bipyramidal structure, all of the fluorine atoms in SF 6 are equivalent. The molecular geometry
is calledoctahedral, because a surface covering the molecule would have eight sides. All of the F-S-F angles are
90° in an octahedral molecule, with the exception of the fluorine atoms that are directly opposite one another which
have a 180° bond angle.
FIGURE 9.9
Central Atom with One or More Lone Pairs
The molecular geometries of molecules change when the central atom has one or more lone pairs of electrons. The
number bonds to the central atom plus the number of lone pairs on the central atom gives us what is called the
electron domain geometry. Electron domain geometries refer to the five molecular shapes learned so far: linear,
trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral. However, if one or more of the bonding pairs of
electrons is replaced with a lone pair, the shape of the molecules is altered. This can apply to any of the geometries
discussed above, but for now we will focus on the tetrahedral electron domain geometry.
Ammonia
The ammonia molecule contains three single bonds and one lone pair on the central nitrogen atom.
The domain geometry for a molecule with four electron pairs is tetrahedral, as was seen with CH 4. In the ammonia
molecule, one of the electron pairs is a lone pair rather than a bonding pair. The molecular geometry of NH 3 is called
trigonal pyramidal.