The Foundations of Chemistry

molecules are only 90°. Most AB 4 molecules are tetrahedral, however, with larger bond
angles (109.5°) and greater separation of valence electron pairs around A.

C. Valence Bond Theory

According to VB theory, each Group IVA atom (C in our example) must make four equiv-
alent orbitals available for bonding. To do this, C forms four sp^3 hybrid orbitalsby mixing
the sand all three porbitals in its outer (n2) shell. This results in four unpaired elec-
trons.

These sp^3 hybrid orbitals are directed toward the corners of a regular tetrahedron, which
has a 109.5° angle from any corner to the center to any other corner.

2 p

sp^3

2 s

C[He] hybridize C[He]

8-7 Tetrahedral Electronic Geometry: AB 4 Species (No Lone Pairs of Electrons on A) 321

Each of the four atoms that bond to C has a half-filled atomic orbital; these can overlap
the half-filled sp^3 hybrid orbitals, as is illustrated for CH 4 and CF 4.

C+C+ C + C C

109.5°

2 p

four sp^3 hybrid

orbitals

2 s

F

F F

F

C

Unshared

pairs on

F atoms are

not shown

H

H

H

H

C

H

1 s 2 p

CH 4 CF 4

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.4, Hybrid Orbitals. Be sure to

see the animation of sp^3 orbital

formation on that screen. See also the

hybridization tool in Screen 10.6,

which you can use to build hybrid

orbitals.

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 10.5, Sigma Bonds.