The Foundations of Chemistry

The Hydrogen Fluoride Molecule, HF

The electronegativity difference between hydrogen (EN2.1) and fluorine (EN4.0)

is very large (
(EN)1.9). The hydrogen fluoride molecule contains a very polar bond

(1.91 D). The bond in HF involves the 1selectron of H and an unpaired electron

from a 2porbital of F. Figure 9-8 shows the overlap of the 1sorbital of H with a 2porbital

of F to form spand sp
molecular orbitals. The remaining two 2porbitals of F have no

net overlap with H orbitals. They are called nonbondingorbitals. The same is true for

the F 2sand 1sorbitals. These nonbonding orbitals retain the characteristics of the

F atomic orbitals from which they are formed. The MO diagram of HF is shown in

Figure 9-9.

Other Diatomic Species with Large
(EN) Values

If the energies of the atomic orbitals of the two atoms of a diatomic molecule or ion are

quite different, the MO diagram may be unlike that known for any homonuclear species.

Its unique MO diagram is constructed by combining the Schrödinger equations for the

two atoms. Construction of the MO diagram for CO is a complex case, beyond the

coverage in this textbook.

Figure 9-8 Formation of spand
sp
molecular orbitals in HF by
overlap of the 1sorbital of H with a
2 porbital of F.

364 CHAPTER 9: Molecular Orbitals in Chemical Bonding

σs p

(bonding)

and

Out-of-phase

overlap (subtract)

In-phase

overlap (add)

Energy

σs p

(antibonding)

Atomic orbitals Molecular orbitals

hydrogen 1s fluorine 2px

2 s

H–F

Molecular

orbitals

H

Atomic

orbitals

F

Atomic

orbitals

σsp

(^)

σsp
1 s
∆E = large
2 p
Nonbonding
orbitals
Nonbonding
orbitals
2 s
1 s
Bonding orbital,
electrons of
highly polar
covalent bond
Bond order = ––––– = 12 – 0 2
Energy
δ+δ–
2 p
1 s
Figure 9-9 MO energy level
diagram for hydrogen fluoride, HF, a
very polar molecule
(1.91 D).
Eis large because
the electronegativity difference is
large.