Chemistry Reference
In-Depth Information
The electron affinity is the energy gained when an electron is added to a neutral gas-phase
atom, and so Mulliken's electronegativities average the ability of a neutral atom to retain
its outermost electrons and its ability to attract an additional electron to form an anion.
The Allred and Rochow and the Mulliken scales place the atoms of the periodic table
in roughly the same order as the Pauling scale; so, for our placement of the AOs in an
MO diagram, the values in Table 7.4 will suffice. The other definitions help to highlight
different approaches to the concept of electronegativity.
We can now use the electronegativity values to place the AO levels at the correct relative
energies before drawing an MO diagram. The LiH case is shown in Figure 7.14; because H
is more electronegative than Li, the H(1s) AO is drawn lower than the 2s or 2p orbitals of
Li. The trend seen in the ionization potentials across the first row in Figure 7.13 suggests
that the 2s and 2p states of Li will be relatively close together, and these levels are placed
accordingly.
σ
+
4
1
π
2p
z
2p
x
2p
y
(
σ
+
)
π
σ
+
3
2s
(
σ
+
)
(
σ
+
)
1s
σ
+
2
H
Li
H
i
σ
+
(
σ
+
)
1
1s
Figure 7.14
The orbitals of LiH. The AOs of H are drawn lower than those of Li due to the
difference in electronegativity. This makes the occupied 2
σ
+
orbital predominantly H-like. In
the MO diagrams, the position of H is shown with a white sphere and that of Li by a dark grey
sphere.
There are no operations which swap atoms over in
C
∞
v
symmetry, there is only the iden-
tity
E
, the infinite number of operations associated with the
C
∞
axis and the infinite number
of vertical mirror planes that contain the molecular axis. Conventionally, we align the
Z
axis with the principal axis, along the molecular bond. The character table in Appendix 12
shows that 1s, 2s and 2p
z
AOs belong to the
σ
+
irreducible representation, while 2p
x
and
2p
y
are degenerate, belonging to the
π
representation.
