NO$_2^-$ ion

Structure (Lewis Structure)

Nitrogen (N) has 2 non-bonding electrons. The total valence electrons of N is 5. Out of 5, 3 electrons are bonding electrons and 2 electrons are non-bonding electrons.

Double bonded oxygen has 4 non-bonding electrons. Oxygen valence electrons : 6

Out of 6, 2 are bonding electrons are 4 are non-bonding electrons.

Negatively charged oxygen has 6 non-bonding electrons. The negative charge is due to the extra electron present. So, the total number of non-bonded electrons present in NO$_2^-$ ion based on Lewis theory is

$$2(of\,N) + 4(of\,O) + 6(of\,{O^ - })$$

$$ = 2 + 4 + 6 = 12$$