The resonance structures are an attempt to make the ion fit the pattern for Lewis diagrams and follow the octet rule. Diagram (I).
The [NO2^-] molecular ion has 1 double bond, 1 single bond, and 6 lone (nonbonding) pair(s) of electrons and 2 resonance forms.
The catch is that NO2^- will have two identical bonds, not a single and a double. Diagram (II) indicates that the two bonds are the same and their bond order of 1.5. In addition, the formal charges assigned to the atoms are closer to zero, with a formal charge of -0.5 assigned to each oxygen atom. Nitrogen has a formal charge of zero. (The sum of the formal charges equals the charge on the ion.)
Ideally.... The [NO2^-] molecular ion has no double bonds, no single bond(s), 2 bonds with bond order of 1.5 and 6 lone (nonbonding) pair(s) of electrons and no resonance forms.
In each, the N has a lone pair (non bonding) electrons and the O has either 3 lone pairs (non bonding) if it is singly bonded, or 2 lone pairs if it is doubly bonded.
So, in summary, the molecule has 1 double bond, 1 single bond, and 6 lone pairs of electrons and 2 resonance forms.
Answers & Comments
Nitrite ion.....
The resonance structures are an attempt to make the ion fit the pattern for Lewis diagrams and follow the octet rule. Diagram (I).
The [NO2^-] molecular ion has 1 double bond, 1 single bond, and 6 lone (nonbonding) pair(s) of electrons and 2 resonance forms.
The catch is that NO2^- will have two identical bonds, not a single and a double. Diagram (II) indicates that the two bonds are the same and their bond order of 1.5. In addition, the formal charges assigned to the atoms are closer to zero, with a formal charge of -0.5 assigned to each oxygen atom. Nitrogen has a formal charge of zero. (The sum of the formal charges equals the charge on the ion.)
Ideally.... The [NO2^-] molecular ion has no double bonds, no single bond(s), 2 bonds with bond order of 1.5 and 6 lone (nonbonding) pair(s) of electrons and no resonance forms.
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O=N-O <-> O-N=O
In each, the N has a lone pair (non bonding) electrons and the O has either 3 lone pairs (non bonding) if it is singly bonded, or 2 lone pairs if it is doubly bonded.
So, in summary, the molecule has 1 double bond, 1 single bond, and 6 lone pairs of electrons and 2 resonance forms.