Chemistry - A Molecular Science

used the energy of the unfilled orbitals to pr

edict relative electronegativities in Chapter 3!

Using the relative energies of atoms X, A, B, and C in Figure 6.24, we conclude that atom C is the most electronegative atom and atom A is the least electronegative. The bond dipole of the AX bond points toward atom X beca

use X is more electronegative than A,

but it points toward atom C in the XC bond because atom C is more electronegative than atom X. Example 6.8

Indicate which representation best describes the interaction of p orbitals in each of the following O-X bonds. Assume the oxygen orbital is on the left in each case.

(a)

(b)

(c)

O-F bond Oxygen is less electronegative than fluorine, so

its p orbital is higher in energy and would

contribute less to the MO. Figure (a) would be the best representation.

Note that the size

of the p orbital in the MO indicates the cont

ribution to the MO, not

the size of the atom.

Oxygen is larger than F, but it

contributes less to the MO, so its p orbital is shown smaller

in the figure. O-N bond Oxygen is more electronegative than nitrogen,

so its p orbital is at lower energy would

contribute more to the MO. Figure (c) would be the best representation. O-O bond The atoms are identical, so their contributions

would be identical as shown in Figure (b).

MOs FOR SIMPLE

π SYSTEMS

So far, we have discussed the MO’s formed by interacting only two atoms, but all of the atoms in a molecule can be involved in a si

ngle MO. Computers are used to determine the

MO’s of complicated molecules, so we limit our discussion to the

MO’s of some simple π

molecules to show how MO theory deals

with delocalized bonds. The discussion also

points out the importance of symmetry in molecular systems.

The number and placement of the nodal planes is the key factor in generating the

(^) π
MOs in the systems we will examine. In th
e following rules for creating MO’s, we use N
to denote the number of AOs used to construct the MOs.
Chapter 6 Molecular Structure & Bonding
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