Biomedical Engineering Reference
In-Depth Information
Fig. 2.6
Total energy of the H
2
molecule as a function of
internuclear separation
R
. A stable molecule is formed when
the spins of the two electrons are antiparallel. A nonbonding
energy level is formed when the spins are parallel. The two
energies coincide at large
R
.
is thus formed by the exchange is called covalent. Resonance also occurs between
the ionic structures (H
A
12
-
,H
B
)and(H
A
21
-
,H
B
) from Fig. 2.5(b) and contributes
∼
5% of the binding energy, giving the H
2
bond a small ionic character. The re-
maining 15% of the binding energy comes from other effects, such as deformation
of the electron wave functions from the simple structures discussed here and from
partial shielding of the nuclear charges by each electron from the other. In general,
for covalent bonding to occur, the two atoms involved must have the same number
of unpaired electrons, as is the case with hydrogen. However, the atoms need not
be identical.
The character of the bond in HF and HCl, for example, is more ionic than in
the homonuclear H
2
or N
2
. The charge distribution in a heteronuclear diatomic
molecule is not symmetric, and so the molecule has a permanent electric dipole
moment. (The two types of bonds are called homopolar and heteropolar.)
Figure 2.6 shows the total energy of the H
2
molecule as a function of the inter-
nuclear separation
R
. (The total energy of the two H atoms at large
R
is taken as the
reference level of zero energy.) The bound state has a minimum energy of
-4.7
eV
at the equilibrium separation of 0.74 Å, in agreement with the data given earlier
in this section. In this state the spins of the bonding electron pair are antiparallel.
