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ScienceQuest

Nitrous acid (HNO₂) Lewis structure

18 valence electrons, bent

N O O H
HNO₂ as the Lewis Structure Generator draws it: lone pairs as dots.

Build HNO₂ step by step in the Lewis Structure Generator

What is the Lewis structure of HNO₂?

Nitrous acid, HNO₂, has 18 valence electrons. Nitrogen is the central atom, joined by one N=O double bond, one N−O single bond and one O−H single bond. Every formal charge is zero. N is AX₂E₁: bent, and the molecule is polar.

Valence electrons in HNO₂

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 1 1
Nitrogen 15 5 1 5
Oxygen 16 6 2 12
Total 18

In one line: 1 + 5 + 2 × 6 = 18. Each atom brings the valence electrons its group gives it.

How to draw the HNO₂ Lewis structure

  1. Count the valence electrons. Valence electrons: H 1, N 5 and each O 6. 1 + 5 + 2 × 6 = 18.
  2. Choose the central atom. N goes in the middle: N is the least electronegative atom other than H, χ 3.04 against 3.44 for O. Checked the other way, O in the middle would need an O−O bond, and electronegative atoms of one element avoid bonding to each other when they can. Each H goes on its own O, as in any acid written with H first.
  3. Join the atoms with single bonds. Three single bonds use 6 electrons, leaving 18 − 6 = 12.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight, H needing nothing more than its bond: O(H) 4 + O 6 = 10, leaving 12 − 10 = 2.
  5. Put the rest on the central atom. The 2 left go on the central atom as one lone pair on N, and N now has 6 electrons round it.
  6. Make multiple bonds where needed. N has only 6 electrons, so one lone pair moves into the bonds, making an N=O bond, and N now has 8.

Lone pairs and formal charges in HNO₂

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
N (central) 1 one double bond and one single bond 1 8 5 − 2 − 3 = 0
O 1 two single bonds 2 8 6 − 4 − 2 = 0
O 1 one double bond 2 8 6 − 4 − 2 = 0
H 1 one single bond 0 2 1 − 0 − 1 = 0

V is the atom’s valence electrons, N its nonbonding electrons and B its bonding electrons. The formal charges add up to 0, the molecule’s overall charge.

Minor resonance contributors to HNO₂

Ranked by the same rules, the next best arrangements are valid Lewis structures that count for less in the real molecule.

N O O H + −
+1 on the O of the O−H and −1 on the single-bonded O. Its formal charges add up to more: 2 against 0.

Shape and polarity of HNO₂

Central atom N
VSEPR class AX₂E₁
Electron geometry trigonal planar
Molecular geometry bent
Ideal bond angle 120°
Polarity, an estimate polar

The O−H groups break the symmetry of the bonds round the center, as in HNO₃, H₂SO₄ and H₃PO₄, which are all polar.

N has two σ bonds and one lone pair, three electron domains in all, so its electron geometry is trigonal planar. Counting only the bonded atoms, the molecular geometry is bent.

The VSEPR Molecular Geometry viewer turns the AX₂E₁ shape in 3D, lone pairs included.

Work with HNO₂