Incomplete Lewis structures for the nitrous acid molecule, HNO2, and the nitrite ion, NO2-, are shown here. (a) Complete each Lewis structure by adding electron pairs as needed. (b) Is the formal charge on N the same or different in these two species? (c) Would either HNO2 or NO2- be expected to exhibit resonance?

The Lewis structure shows electron pairs in bonds between the atoms and the solitary electron pairs that remain for each atom. (a) in attached files, (b) the formal charge on N is the same +3, (c) NO₂⁻ exhibits resonance  

Nitrous acid molecule, HNO₂

N has 5 electrons

H has 1 electron

O has 6 electrons

There are two single bonds between H, O, and N

There is one double bond between N and the other O

Each bond represents two electrons

Excepting H, the other elements need to have eight electrons in total.

5 eletrons from N + 1 eletron from H + 12 electrons from the two O = 18 e⁻

18 e⁻ = 9 pairs or electrons

The two simple bonds + the double bond represent 4 electron pairs. So, there are still 5 electron pairs to place.

One of the O forms two single bonds, so it already has 2 pairs of electrons. It needs two more pairs to get the 8e⁻. So 4 electrons are placed arround this O.

The other O forms a double bond with N. The double bond represents 4 electrons. So this O needs another 4 electrons to get to the octet. Two pairs are placed arround this atom.

N forms a single bond with one O and a double bond with another O, so it already has 6 electrons. It only needs 2 more electrons to get to the octet. One pair is placed over the atom.

Finally H does not need to get any other electron

Nitrite ion, NO₂⁻

N has 5 electrons

O has 6 electrons

And there is a free negative charge, e⁻

There is one single bond between one of the O, and N

There is one double bond between N and the other O

The double bond is composed of a  single bond and a resonant bond

Each bond represents two electrons

5 eletrons from N + 12 electrons from the two O + 1e⁻ = 18 e⁻

18 e⁻ = 9 pairs or electrons

The single bonds + the double bond represent 3 electron pairs. So, there are still 6 electron pairs to place.

One of the O forms one single bond, so it already has 2 electrons. It needs three more pairs to get the 8e⁻. So 6 electrons are placed arround this O.

The other O forms a double bond with N. The double bond represents 4 electrons. So this O needs another 4 electrons to get to the octet. Two pairs are placed arround this atom.

N forms a single bond with one O and a double bond with another O, so it already has 6 electrons. It only needs 2 more electrons to get to the octet. One pair is placed over the atom.      

b) The formal charge on N (= +3) is the same for both species  

The formal charges refer to the partial charges of each atom that are shared with another atom. It is the difference between the total valence electrons (8), the electrons that are not in the bond (2), and half the shared electrons (3).    

formal charge on N = 8 - 2 - 3 = +3

c) Since nitrite ion, NO2- is the only one that exhibits a symetrical structure, it is the only one expected to show resonance.

You can learn more about the Lewis structure at

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