The Foundations of Chemistry

The results that we discussed in Sections 8-5 (BeX 2 , CdX 2 , and HgX 2 , where XCl,

Br, or I), and 8-6 (BX 3 , where XF, Cl, Br, or I), and in this section (CH 4 , CF 4 , CCl 4 ,

SiH 4 , SiF 4 , NH 4 , and SO 42 
) illustrate an important generalization:

When a molecule or polyatomic ion has no unshared pairs of valence electrons on

the central atom, the electronic geometryand the molecular geometryare the same.

Examination of bond dipoles shows that in CH 4 the individual bonds are only slightly

polar, whereas in CF 4 the bonds are quite polar. In CH 4 the bond dipoles are directed

toward carbon, but in CF 4 they are directed away from carbon. Both molecules are very

symmetrical, so the bond dipoles cancel, and both molecules are nonpolar. This is true

for all AB 4 molecules in which there are no unshared electron pairs on the central elementand

all four B atoms are identical.

320 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

In some tetrahedral molecules, the atoms bonded to the central atom are not all the

same. Such molecules are usually polar, with the degree of polarity depending on the rela-

tive sizes of the bond dipoles present. In CH 3 F or CH 2 F 2 , for example, the addition of

unequal dipoles makes the molecule polar.

The VSEPR theory also predicts that NH 4 and SO 42 
ions have tetrahedral elec-

tronic geometry. Each region of high electron density bonds the central atom to another

atom (H in NH 4 , O in SO 42 
) at the corner of the tetrahedral arrangement. We describe

the molecular geometry of each of these ions as tetrahedral.

You may wonder whether square planar AB 4 molecules exist. We will discuss some

examples of square planar AB 4 species in Section 8-12. The bond angles in square planar

CF 4

C F

EN2.5 4.0

(EN)1.5

Net molecular

dipole 0

C

H

F

F

C

F

F

H

H

H

Net molecular

dipole 0

CH 4

C H

EN2.5 2.1

(EN)0.4

F

C

F

C

H Net molecular H

dipole > 0

Net molecular

dipole > 0

H

H

H

F