# What is the resonance structure for NO_2^-?

$O = N - {O}^{-}$. This is a bent molecule, with a lone pair of electrons on the central nitrogen.
There are 18 electrons to distribute over 3 centres (i.e. $2 \times 6 + 5 + 1$), and an associated formal negative charge.
Clearly, the alternative structure ""^(-)O-N=O is equivalent, And the geometry of the nitrogen lone pair, and the $2$ $N - O$ bonds are trigonal planar to a first approximation.
$\angle O - N - O < {120}^{\circ}$ due to the influence of the nitrogen lone pair.