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Hypophosphorous acid (H₃PO₂) Lewis structure

20 valence electrons, tetrahedral

P O O H H H
H₃PO₂ as the Lewis Structure Generator draws it: lone pairs as dots.

Build H₃PO₂ step by step in the Lewis Structure Generator

What is the Lewis structure of H₃PO₂?

Hypophosphorous acid, H₃PO₂, has 20 valence electrons. Phosphorus is the central atom, joined by one P=O double bond, one P−O single bond, two P−H single bonds and one O−H single bond. Every formal charge is zero. P is AX₄: tetrahedral, and the molecule is polar.

Valence electrons in H₃PO₂

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 3 3
Phosphorus 15 5 1 5
Oxygen 16 6 2 12
Total 20

In one line: 3 × 1 + 5 + 2 × 6 = 20. Each atom brings the valence electrons its group gives it.

How to draw the H₃PO₂ Lewis structure

  1. Count the valence electrons. Valence electrons: each H 1, P 5 and each O 6. 3 × 1 + 5 + 2 × 6 = 20.
  2. Choose the central atom. Hypophosphorous acid is H₂PO(OH), with two H on P (Greenwood and Earnshaw, 1997, chapter 12).
  3. Join the atoms with single bonds. Five single bonds use 10 electrons, leaving 20 − 10 = 10.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight, H needing nothing more than its bond: O 6 + O(H) 4 = 10, leaving 10 − 10 = 0.
  5. Put the rest on the central atom. Nothing is left, so P gets no lone pair, and P now has 8 electrons round it.
  6. Make multiple bonds where needed. P already has an octet, but turning one lone pair into a P=O bond lowers its formal charge from +1 to 0. That is what the smallest formal charges convention asks for; octets first stops a step earlier.

Lone pairs and formal charges in H₃PO₂

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
P (central) 1 one double bond and three single bonds 0 10 5 − 0 − 5 = 0
O 1 one double bond 2 8 6 − 4 − 2 = 0
O 1 two single bonds 2 8 6 − 4 − 2 = 0
H 3 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.

H₃PO₂ with an octet on every atom

The structure above keeps the formal charges as small as they go and puts 10 electrons round P. Keeping every atom to eight instead, the octets-first convention, gives one form: two P−O single bonds, two P−H single bonds and one O−H single bond, with formal charges of +1 on the central P and −1 on the single-bonded O. Courses differ on which of the two they expect.

P O O H H H + −
Octets first

Draw H₃PO₂ with octets first in the Lewis Structure Generator

Shape and polarity of H₃PO₂

Central atom P
VSEPR class AX₄
Electron geometry tetrahedral
Molecular geometry tetrahedral
Ideal bond angle 109.5°
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.

P has four σ bonds and no lone pairs, four electron domains in all, so its electron geometry is tetrahedral. With no lone pairs, the molecular geometry is the same.

The VSEPR Molecular Geometry viewer turns the AX₄ shape in 3D, lone pairs included.

Work with H₃PO₂