Page 123 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:37 9in x 6in b985-ch10 Elementary Physical Chemistry

108 Elementary Physical Chemistry

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Diagram representing sp hybrids in CH 2=CH 2.
Fig. 10.3
resulting in two σ bonds that should be 90 apart. However, the bond angle
◦
◦
turns out to be 104 .
Another example is NH 3 . The valence electron configuration of N, as
noted before, has 3 unpaired electrons, namely 2p x,2p y and 2p z and thus
can form 3 σ bonds with H which should be perpendicular to each other.
However, each of the experimental angles of the pyramidal molecule is 107 .
◦
Thus, the valence bond theory is not a good predictor of the bond
angles. But where valence bond theory is particularly deficient is in pre-
dicting the shape of the molecule CH 4 . The valence electron configuration
2
1
1
◦
of C is 2s 2p 2p and should have 2 bonds separated by 90 .But CH 4 has
z
y
4 bonds about 109 apart.
◦
To overcome this deficiency, two additional concepts had to be intro-
duced, the concept of promotion and the concept of hybridization. Speci-
fically, in the case of CH 4 ,one of the2s electrons is promoted to a 2p state,
1
1
1
1
1
forminga2p orbital, thus giving rise to the C configuration 2s 2p 2p 2p .
z
x
x
y
These orbitals form four hybrid orbitals, each consisting of the combination
3
of 1s and 3p’s. These orbitals are called sp orbitals. Each of these hybrid
orbitals consists of a small and a large lobe. The large lobes point towards
the corners of a tetrahedron.
Other hybrid orbitals have been introduced to explain other molecules,
for example ethylene, CH 2 =CH 2 (Fig. 10.3). Ethylene makes use of sp 2
orbitals which are planar and 120 apart.
◦
Sometimes d orbitals must be included to explain certain structures, for
example, PCl 5 .Even sp hybrids have been used to explain some structures,
for example, HC≡CH.
10.3. Molecular Orbital Theory
It is assumed here that the electrons spread over the entire molecule, rather
than belonging to a particular bond. The essential features of molecular
+
orbital theory are illustrated by H which has an exact solution.
2