The Foundations of Chemistry

A. Experimental Facts and Lewis Formulas

Each Group VA element has five electrons in its valence shell. The Group VA elements

form some covalent compounds by sharing three of those electrons with three other atoms.

Let us describe two examples: ammonia, NH 3 , and nitrogen trifluoride, NF 3. Each is a

trigonal pyramidal, polar molecule with an unshared pair on the nitrogen atom. Each has

a nitrogen atom at the apex and the other three atoms at the corners of the triangular

base of the pyramid.

The Lewis formulas for NH 3 and NF 3 are

Sulfite ion, SO 32 
, is an example of a polyatomic ion of the AB 3 U type. It is a trigonal

pyramidal ion with an unshared pair on the sulfur atom.

B. VSEPR Theory

As in Section 8-7, VSEPR theory predicts that the fourregions of high electron density

around a central atom will be directed toward the corners of a tetrahedron, because this

gives maximum separation. So N has tetrahedral electronic geometry in NH 3 and NF 3.

We must reemphasize the distinction between electronic geometry and molecular geom-

etry. Electronic geometryrefers to the geometric arrangement of the regions of electron density

around the central atom. But the molecular geometryexcludes the unshared pairs on the

central atom, and describes only the arrangement of atoms(i.e., nuclei) around the central

atom. We can represent the tetrahedral electronic geometry around N in NH 3 or NF 3 as

follows.

O

O S O

2 

F

H H FFN

H

N

NH 3 NF 3

Some Group VA elements also form
covalent compounds by sharing all five
valence electrons (Sections 7-7 and
8-11).

324 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

We then use the Lewis formula as a guide to put the bonded atoms and the lone pairs in

these tetrahedral sites around the nitrogen atom.

H

N

H

H F

N

F

F

N