The Foundations of Chemistry

exerting strong attractive forces on it, so it resides closer to the nucleus than do bonding

electrons. The relative magnitudes of the repulsive forces between pairs of electrons on

an atom are

lp/lplp/bpbp/bp

where lprefers to lone (unshared) pairs and bprefers to bonding pairs of valence shell

electrons. We are most concerned with the repulsions among the electrons in the valence

shell of the central atomof a molecule or polyatomic ion. The angles at which repulsive

forces among valence shell electron pairs are minimized are the angles at which the

bonding pairs and unshared pairs (and therefore nuclei) are found in covalently bonded

molecules and polyatomic ions. Due to lp/bprepulsions in NH 3 and NF 3 , their bond angles

are lessthan the angles of 109.5° we observed in CH 4 and CF 4 molecules.

In NH 3 the bond dipoles reinforcethe
effect of the unshared pair, so NH 3 is
very polar (1.47 D). In NF 3 the
bond dipoles opposethe effect of the
unshared pair, so NF 3 is only slightly
polar (0.23 D).

326 CHAPTER 8: Molecular Structure and Covalent Bonding Theories

The formulas are frequently written as SNH 3 and SNF 3 to emphasize the unshared

pairs of electrons. The unshared pairs must be considered as the polarities of these mole-

cules are examined; they are extremely important in chemical reactions. This is why NH 3

is a base, as we saw in Section 4-2.4 and as we shall discuss more fully in Chapter 10. The

contribution of each unshared pair to polarity can be depicted as shown in the margin.

The electronegativity differences in NH 3 and NF 3 are nearly equal, butthe resulting

nearly equal bond polarities are in opposite directions.

Thus, we have

N H

3.0 2.1 N H

0.9



(EN) 

EN

NF

3.0 4.0 N F

1.0



(EN) 

EN

We can now use this information to explain the bond angles observed in NF 3 and NH 3.

Because of the direction of the bond dipoles in NH 3 , the electron-rich end of each NXH

bond is at the central atom, N. On the other hand, the fluorine end of each bond in NF 3

is the electron-rich end. As a result, the lone pair can more closely approach the N in

Tetrahedral

electronic geometry

Trigonal pyramidal

molecular geometry

In NF 3 , F–N–F angle = 102.1°

F

F

N

F

In NH 3 , H–N–H angle = 107.3°

H

H N

H

F

F

N

F

Net molecular

dipole > 0 (small)

N

H

H

H

Net molecular

dipole > 0 (large)

A model of NH 3 , a trigonal
pyramidal AB 3 U molecule, showing
the lone pair.

H

H H

N

107.3°