Exception to Octet Rule (Chemical Bonds: 11)

Course Chemical Bonds: Covalent Bonding and Shapes of Molecules (11)

Exception to octet rule:

We mentioned that many compounds follow the octet rule, in which each atom is surrounded by 8 electrons.

First Exception

Although the octet rule is successful with organic compounds, some inorganic compounds exhibit discrepancies and deviations from the octet rule. Elements in Group 3A are among those that do not follow the octet rule; they have only three valence electrons. When one of these elements serves as the central atom, it displays fewer electrons than the octet rule’s requirement of eight electrons.

Boron Trichloride

Boron has only three valence electrons.

Boron valence electrons =+3

Boron in B(Cl)₃ surrounded by 3 bonds = 6 e’s

Aluminium Trichloride

Another example: aluminum has thirteen electrons, with two in the first level, eight in the second level, and three in the third level.

Aluminum’s valence electrons = +3

In aluminum trichloride, aluminum forms three bonds with the three chlorine atoms, leaving aluminum surrounded by only six electrons

Second Exception

A second reason for compounds not following the octet rule is the presence of an odd number of electrons.

Nitrogen Oxide

For example, nitric oxide contains nitrogen, which is doubly bonded with oxygen. We know that each bond represents two electrons, meaning oxygen has two bonds with four electrons and four unpaired electrons, totaling eight electrons.

For nitrogen, with 5 valence electrons, there will be three unbounded electrons. This leaves the nitrogen with only seven surrounding electrons. We can’t fit this odd electron.

Nitrogen has 4 bonding e’s and 3 non-bonding e’s = 7e’s

Ammonia

But you need to remember, not all nitrogen compounds disobey the octet rule. For example, in ammonia, nitrogen is surrounded by eight electrons—six bonded electrons and two nonbonding electrons.

Nitrogen has 6 bonding e’s and 2 non-bonding e’s = 8e’s

The Third Exception:

The third exception to the octet rule occurs when the central atom in molecules has more than 8 electrons due to like phosphorus or sulfur, can accommodate more than eight electrons due to the available d orbital. For example, the phosphorus atom has five valence electrons.

Phosphorus

The phosphorus atom can form five bonds with five fluorine atoms, resulting in a total of ten surrounding electrons, which exceeds what the octet rule permits. The phosphorus atom can accommodate the extra electrons in the d orbital.

Phosphorus Hexafluoride

Phosphorus atom surrounded by 5 bonds = 10 bonding e’s

Three electrons pair with 3p orbital, the extra electrons fit in 3d orbital.

Sulfur Hexafluoride

Similar to phosphorus, sulfur forms sulfur hexafluoride in which sulfur is surrounded by twelve electrons. It is surrounded by six bonds with twelve bonding pairs.

Sulfur surrounded by 6 bonds = 12 bonding e’s

Sulfur Difluoride

Sulfur can obey the octet rule in some compounds, such as sulfur difluoride. In this compound, sulfur is bonded to two fluoride atoms, with four bonding electrons, and has two unbonded lone pairs, adding another four electrons, for a total of eight electrons.

Sulfur has 4bonding e’s and 4 nonbonding e’s.

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Covalent Bonding and Shape of Molecules

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