The Foundations of Chemistry

Then we describe the molecular geometryas the arrangement of the atoms.In each of these
molecules, the N atom is at the apex of a (shallow) trigonal pyramidal arrangement and
the other three atoms are at the corners of the triangular base of the pyramid. Thus, the
molecular geometry of each molecule is described as trigonal pyramidal.

8-8 Tetrahedral Electronic Geometry: AB 3 U Species (One Lone Pair of Electrons on A) 325

We have seen that CH 4 , CF 4 , NH 3 , and NF 3 all have tetrahedral electronic geometry.
But CH 4 and CF 4 (AB 4 ) have tetrahedral molecular geometry, whereas NH 3 and NF 3
(AB 3 U) have trigonal pyramidal molecular geometry.

In molecules or polyatomic ions that contain lone (unshared) pairs of valence elec-

trons on the central atom, the electronic geometryand the molecular geometrycannot

be the same.

Because this trigonal pyramidal molecular geometry is a fragment of tetrahedral elec-
tronic geometry, we expect that the HXNXH angle would be close to the tetrahedral
value, 109.5°. In CH 4 (a tetrahedral AB 4 molecule), all HXCXH bond angles are observed
to be this ideal value, 109.5°. In NH 3 , however, the HXNXH bond angles are observed
to be less than this, 107.3°. How can we explain this deviation?
A lone pair is a pair of valence electrons that is associated with only one nucleus in
contrast to a bonded pair, which is associated with two nuclei. The known geometries of
many molecules and polyatomic ions, based on measurements of bond angles, show that
lone pairs of electrons occupy more space than bonding pairs.A lone pair has only one atom

H

N

H

H

F

N

F

F

C

H

H H

H

109.5°