http://www.ck12.org Chapter 9. Covalent Bonding
Unlike the other basic shapes, the five chlorine atoms in this arrangement are not equivalent with respect to their
geometric relationship to the phosphorus atom. Three of the chlorine atoms lie in a plane, with Cl-P-Cl bond angles
of 120°. This portion of the molecule is essentially the same as a trigonal planar arrangement. These chlorine atoms
are referred to as theequatorialatoms because they are arranged around the center of the molecule. The other two
chlorine atoms are oriented exactly perpendicular to the plane formed by the phosphorus atom and the equatorial
chlorine atoms. These are called theaxialchlorine atoms.
FIGURE 9.14
(left) Trigonal bipyramidal. (right) Ball and
stick model of phosphorus pentachloride.In the figure above (Figure9.14), the axial chlorine atoms form a vertical axis with the central phosphorus atom.
There is a 90° angle between P-Claxialbonds and P-Clequitorialbonds. The molecular geometry of PCl 5 is called
trigonal bipyramidal. A surface covering the molecule would take the shape of two three-sided pyramids pointing
in opposite directions.
AB
The sulfur atom in sulfur hexafluoride also exceeds the octet rule.
Unlike the trigonal bipyramidal structure, all of the fluorine atoms in SF 6 are equivalent. The molecular geometry is
calledoctahedral(Figure9.15) 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.
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
total number of electron pairs, both bonding pairs and lone pairs, leads to what is called theelectron domain
geometry. Electron domain geometries are one of the five learned so far: linear, trigonal planar, tetrahedral, trigonal