Question

What is the formal charge of ${NO}_2^{-}$?

Answer 1

Hint: The formal charge is the difference in the number of valence electrons in the atom and the number of valence electrons in the Lewis structure.

Complete answer step-by-step:
The formal charge of an atom in a molecule can be calculated by the following equation;
$FC=V-N-B÷2$
where V= number of valence electrons of the atom in the free atom
N = number of nonbonding electrons on this atom in the molecule
B = Total number of electrons shared in covalent bonds with other atoms in the molecule.

Nitrogen in the nitro group ${NO}_2^{-}$ : FC= $5-2-6÷2=0$
Double bonded oxygen in ${NO}_2^{-}$ : FC=$6-4-4÷2=0$
Single bonded oxygen in ${NO}_2^{-}$ : FC= $6-6-2÷2=-1$
Hence, $0+0+(-1)=-1$ as expected for ${NO}_2^{-}$

Additional Information:
Formal charge means ignore the lone pair of electrons and electrons shared by that atom from valence electrons then whatever is left with you is the significance of formal charge.
- It helps us to keep track of the electrons in a molecule.
- It tells you whether an atom has more electrons or protons associated with it.
- It allows us to assign electrons to a particular atom in a molecule.

Note: The possibility to make a mistake is that you may confuse between formal charge and the actual charge. But keep in mind that formal charges do not represent the actual charge on atoms in a molecule. The formal charges are assigned to the atoms in the Lewis structure to keep track of the electrons involved in the bonding.