The Foundations of Chemistry

We can imagine that there is one electron in each of these hybrid orbitals. Each of the

three F atoms has a 2porbital with one unpaired electron. The 2porbitals can overlap

the three sp^2 hybrid orbitals on B. Three electron pairs are shared among one B and three

F atoms:

The names of many solid figures are
based on the numbers of plane faces
they have. A regulartetrahedron is a
three-dimensional figure with four
equal-sized equilateral triangular faces
(the prefix tetra- means “four”).

318 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

sp^2 Hybridization occurs at the central atom whenever there are three regions of

high electron density around the central atom. AB 3 molecules and ions with no

unshared pairs on the central atom have trigonal planar electronic geometry, trig-

onal planar molecular geometry, and sp^2 hybridization on the central atom.

TETRAHEDRAL ELECTRONIC GEOMETRY: AB 4 SPECIES

(NO LONE PAIRS OF ELECTRONS ON A)

A. Experimental Facts and Lewis Formulas

Each Group IVA element has four electrons in its highest occupied energy level. The

Group IVA elements form many covalent compounds by sharing those four electrons with

four other atoms. Typical examples include CH 4 (mp 
182°C), CF 4 (mp 
184°C), CCl 4

(mp 
23°C), SiH 4 (mp 
185°C), and SiF 4 (mp 
90°C). All are tetrahedral, nonpolar

molecules (bond angles109.5°). In each, the Group IVA atom is located in the center

of a regular tetrahedron. The other four atoms are located at the four corners of the tetra-

hedron.

The Group IVA atom contributes four electrons to the bonding in a tetrahedral AB 4

molecule, and the other four atoms contribute one electron each. The Lewis formulas for

methane, CH 4 , and carbon tetrafluoride, CF 4 , are typical.

The ammonium ion, NH 4 , and the sulfate ion, SO 42 
, are familiar examples of poly-

atomic ions of this type. In each of these ions, the central atom is located at the center of

F

F

FFC

H

H

HHC

CH 4 , methane CF 4 , carbon tetrafluoride

8-7

sp^2 hybrid orbitals

2 p

F

F F

B

Lone pairs of electrons are not shown
for the F atoms.