[Indranil (OP)]

Why NO₂- ion got the negative charge?
If we see the structure of  NO₂-, each atom in this ion has full filled their octet. So, why there is a negative charge on this ion?

[Kryptid]

In order to have the octet of each atom filled, an extra electron is required. Here is a website that shows the structure of the nitrite ion. Count the number of valence electrons that each atom contributes: https://chemfiesta.org/2015/09/18/resonance-structures/

[Indranil (OP)]

In order to have the octet of each atom filled, an extra electron is required. Here is a website that shows the structure of the nitrite ion. Count the number of valence electrons that each atom contributes: https://chemfiesta.org/2015/09/18/resonance-structures/

From where does this ion get an extra electron?

[NeilT]

From the associated cation. If you take the case of nitrous acid then it takes it from the hydrogen, or for sodium nitrite then it takes it from the sodium.