Carbon monoxide

CO · carbon monoxide

A triple bond plus a lone pair on each atom satisfies both octets, at the cost of formal charges (C −1, O +1), a classic example of octet completion outweighing formal-charge minimization.

Valence electrons: 10
Charge: 0
Lone pairs: 2
Molecular geometry: linear

Lines represent covalent bonds (shared electron pairs); dots represent lone pairs (nonbonding electrons).
A single bond is one shared pair of electrons, a double bond two shared pairs, and a triple bond three shared pairs.
Each atom in a bond contributes to the shared pair and counts the shared electrons toward its own octet. The atoms do not each gain two extra electrons; they share the same pair, and both atoms count it.
The small red + and blue − numbers beside atoms are , not the ion's overall charge. The small numbered circles just tell identical atoms apart, so “O #2” in the table is the same oxygen as “2” in the drawing.

Count

A must account for exactly the , so total them first: carbon is a Group 14 element, so it has 14 - 10 = 4 valence electrons; oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, giving 10 valence electrons that the finished structure has to use in full, with none added and none left over.

Build

With only two , there is no to select: C and O are bonded directly to each other.

Verify

Before continuing, check the total electron count surrounding each . C: 6 shared + 2 nonbonding = 8 (a full ); O: 6 shared + 2 nonbonding = 8 (a full octet).
Determine the of each (, minus lone-pair electrons, minus half of the ): C = -1, O = +1. Their sum is 0, matching the species' overall charge. Formal charge is a bookkeeping device rather than a real ionic charge, and the negative formal charge is placed on the most electronegative atom, which is where it is most stable, so this arrangement is the preferred structure.

Watch out

If either in CO comes up short, keep sharing: it takes a before both counts land where they should. More sharing costs the budget nothing.

Shape

The is linear: two necessarily lie along a single line.
: O pulls harder on the ( gap about 0.9), so the O end carries a partial negative charge. This is a polar .

Worth knowing

That is one of the strongest bonds known, about 1072 kJ/mol. Carbon monoxide is dangerous for a related reason: it grips the iron in your hemoglobin about 200 times more tightly than oxygen does.

Count

A must account for exactly the , so total them first: carbon is a Group 14 element, so it has 14 - 10 = 4 valence electrons; oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, giving 10 valence electrons that the finished structure has to use in full, with none added and none left over.

Verify

Before continuing, check the total electron count surrounding each . C: 6 shared + 2 nonbonding = 8 (a full ); O: 6 shared + 2 nonbonding = 8 (a full octet).

Determine the of each (, minus lone-pair electrons, minus half of the ): C = -1, O = +1. Their sum is 0, matching the species' overall charge. Formal charge is a bookkeeping device rather than a real ionic charge, and the negative formal charge is placed on the most electronegative atom, which is where it is most stable, so this arrangement is the preferred structure.

Watch out

If either in CO comes up short, keep sharing: it takes a before both counts land where they should. More sharing costs the budget nothing.

Shape

The is linear: two necessarily lie along a single line.

: O pulls harder on the ( gap about 0.9), so the O end carries a partial negative charge. This is a polar .

Worth knowing

That is one of the strongest bonds known, about 1072 kJ/mol. Carbon monoxide is dangerous for a related reason: it grips the iron in your hemoglobin about 200 times more tightly than oxygen does.