Nitric oxide

NO · nitric oxide

Eleven valence electrons, an odd number, force one unpaired electron. It sits on nitrogen (the less electronegative atom) to minimize formal charge, making NO a reactive free radical.

Valence electrons: 11
Charge: 0
Lone pairs: 3
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.

Count

Start from the -electron total, because it sets how many electrons the whole structure may contain: nitrogen is a Group 15 element, so it has 15 - 10 = 5 valence electrons; oxygen is a Group 16 element, so it has 16 - 10 = 6 valence electrons, for 11 valence electrons in all; finishing with more or fewer than 11 would mean a counting error was made.

Build

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

Verify

Verify each in turn, counting shared and together. N: 4 shared + 3 nonbonding = 7 (7 electrons); O: 4 shared + 4 nonbonding = 8 (a full ).
Evaluate the of every , defined as minus minus one half of the . Lone-pair electrons count in full because they belong to one atom, while bonding electrons are divided evenly between the two bonded atoms. Every atom in this structure comes out to zero, so no atom bears an artificial charge and the structure is favored.
Because the -electron total is odd, complete pairing is impossible and a single remains on N. This defines a free radical; the lone unpaired electron is the reason radicals react readily with most other molecules.

Watch out

With an odd electron count, one electron must stay unpaired. The common error is inventing an extra electron to tidy the picture up: count what you have, not what you wish you had.

Shape

With only two , N and O define a straight line, so the is linear.
: the gap between N and O is only about 0.4, too small to matter. Effectively nonpolar.

Worth knowing

An odd electron count means NO is a , and this particular radical won a Nobel Prize: your blood vessels use nitric oxide as a signal to relax and widen.