http://www.ck12.org Chapter 9. Covalent Bonding
FIGURE 9.7
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 9.8
(left) Trigonal bipyramidal. (right) Ball and
stick model of phosphorus pentachloride.In theFigure9.8, 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 calledtrigonal bipyramidal. A
surface covering the molecule would take the shape of two three-sided pyramids pointing in opposite directions.
AB 6
The sulfur atom in sulfur hexafluoride (SF 6 ) also exceeds the octet rule.