The Foundations of Chemistry

The sulfur atom in the sulfite ion, SO 32 
, can be described as sp^3 hybridized. One of

these hybrid orbitals contains the sulfur lone pair, and the remaining three overlap with

oxygen orbitals to form bonds.

AB 3 U molecules and ions, each having four regions of high electron density around

the central atom, usuallyhave tetrahedral electronic geometry, trigonal pyramidal

molecular geometry, and sp^3 hybridization on the central atom.

We must remember that theory(and its application) depends on fact, not the other way

around. Sometimes the experimental facts are not consistent with the existence of hybrid

orbitals. In PH 3 and AsH 3 , each HXPXH bond angle is 93.7°, and each HXAsXH bond

angle is 91.8°. These angles very nearly correspond to three porbitals at 90° to each other.

Thus, there appears to be no need to use the VSEPR theory or hybridization to describe

the bonding in these molecules. In such cases, we just use the “pure” atomic orbitals rather

than hybrid orbitals to describe the bonding.

328 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

Problem-Solving Tip:When Do We Not Need To Describe Hybrid

Orbitals?

Students often wonder how to recognize when hybridization does not apply, as in the

PH 3 and AsH 3 cases just described and in H 2 S (see Section 8-9). Remember that models

such as hybridization are our attempts to explain observations such as bond angles. At

the level of our studies, we will be given information about the observed molecular geom-

etry, such as measured bond angles. If these are near the angles of pure (unhybridized)

orbitals, then hybridization is not needed; if they are near the predicted angles for

hybridized orbitals, then hybridization should be used in our explanation. If no infor-

mation about observed molecular geometry (molecular shape, bond angles, etc.) is

supplied, you should assume that the VSEPR and hybridization approaches presented in

this chapter should be used.

TETRAHEDRAL ELECTRONIC GEOMETRY: AB 2 U 2 SPECIES

(TWO LONE PAIRS OF ELECTRONS ON A)

A. Experimental Facts and Lewis Formulas

Each Group VIA element has six electrons in its valence shell. The Group VIA elements

form many covalent compounds by sharing a pair of electrons with each of two other

atoms. Typical examples are H 2 O, H 2 S, and Cl 2 O. The Lewis formulas for these mole-

cules are

All are angular, polar molecules. The bond angle in water, for example, is 104.5°, and the

molecule is very polar with a dipole moment of 1.85 D.

H O

H

H S

H

Cl O

Cl

H 2 OH 2 SCl 2 O

8-9