Chemistry · Ch 4 — Chemical Bonding and Molecular Structure

Electronic Configuration and Molecular Behaviour

4.7.5

Electronic Configuration and Molecular Behaviour

The distribution of electrons among the various molecular orbitals of a molecule is called its electronic configuration. Once you know this configuration, you can extract several important properties of the molecule — its stability, the nature of its bonds, its bond length, and its magnetic behaviour. Each of these follows directly from the numbers of electrons in bonding and antibonding orbitals.

Stability of Molecules

Let $N_b$ be the number of electrons occupying bonding molecular orbitals, and $N_a$ the number occupying antibonding molecular orbitals. The stability of the molecule is decided by a simple comparison:

(I) The molecule is stable if $N_b > N_a$ .
(II) The molecule is unstable if $N_b < N_a$ .

Why does this work? In case (I), more bonding orbitals are occupied than antibonding ones. The bonding influence — the net attractive force that holds the atoms together — is stronger, so a stable molecule results. In case (II), the antibonding influence dominates, making the molecule unstable. If $N_b = N_a$ , the bonding and antibonding effects cancel exactly, and no stable molecule forms.

Note

This rule applies to diatomic molecules and simple polyatomic species where molecular orbital theory is directly applicable. It is the foundation for everything that follows.

Bond Order

Bond order ( $b.o.$ ) is defined as one half the difference between the number of bonding electrons and the number of antibonding electrons:

\text{Bond order} = \frac{1}{2}(N_b - N_a)

The stability rules above can be restated in terms of bond order:

A positive bond order ( $N_b > N_a$ ) means a stable molecule.
A negative bond order ( $N_b < N_a$ ) or zero bond order ( $N_b = N_a$ ) means an unstable molecule.

Watch out

A negative bond order does not mean a "negative bond" — it simply indicates that the molecule is less stable than the separated atoms. Such species are not observed under normal conditions.

Nature of the Bond

The bond order value tells you the type of bond between two atoms, matching the classical concept of single, double, and triple bonds:

Bond order = 1 → single bond
Bond order = 2 → double bond
Bond order = 3 → triple bond

These are integral values. In practice, bond orders can also be fractional (e.g., 1.5 in benzene or 2.5 in $\text{O}_2^+$ ), indicating resonance or partial bonding.

Bond Length

Bond order serves as an approximate measure of bond length. The relationship is inverse:

Important

Bond length decreases as bond order increases.

For example, a triple bond (bond order 3) is shorter than a double bond (bond order 2), which in turn is shorter than a single bond (bond order 1). This makes physical sense: more bonding electrons pull the nuclei closer together.

Tip

When comparing bond lengths in a series of related molecules or ions, always check the bond order first. The higher the bond order, the shorter and stronger the bond.

Magnetic Nature

The magnetic behaviour of a molecule depends on whether its electrons are paired or unpaired:

If all molecular orbitals are doubly occupied (every electron is paired), the substance is diamagnetic — it is weakly repelled by a magnetic field. …

Start your 14-day free trial to unlock the full solution →

← Previous · 4.7.4Energy Level Diagram for Molecular Orbitals Next · 4.8 →Bonding in Some Homonuclear Diatomic Molecules