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Phosphoric acid (H₃PO₄) Lewis structure

32 valence electrons, tetrahedral

P O O O O H H H
H₃PO₄ as the Lewis Structure Generator draws it: lone pairs as dots.

Build H₃PO₄ step by step in the Lewis Structure Generator

What is the Lewis structure of H₃PO₄?

Phosphoric acid, H₃PO₄, has 32 valence electrons. Phosphorus is the central atom, joined by one P=O double bond, three P−O single bonds and three O−H single bonds. It carries two lone pairs on each O of an O−H and two on the double-bonded O. Every formal charge is zero. P is AX₄: tetrahedral, and the molecule is polar.

Valence electrons in H₃PO₄

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 3 3
Phosphorus 15 5 1 5
Oxygen 16 6 4 24
Total 32

In one line: 3 × 1 + 5 + 4 × 6 = 32. Each atom brings the valence electrons its group gives it.

How to draw the H₃PO₄ Lewis structure

  1. Count the valence electrons. Valence electrons: each H 1, P 5 and each O 6. 3 × 1 + 5 + 4 × 6 = 32.
  2. Choose the central atom. P goes in the middle: P is the least electronegative atom other than H, χ 2.19 against 3.44 for O. Each H goes on its own O, as in any acid written with H first.
  3. Join the atoms with single bonds. Seven single bonds use 14 electrons, leaving 32 − 14 = 18.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight, H needing nothing more than its bond: 3 O(H) × 4 + O 6 = 18, leaving 18 − 18 = 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 H₃PO₄

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 3 two single bonds 2 8 6 − 4 − 2 = 0
O 1 one double bond 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.

H₃PO₄ 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: four P−O single bonds and three O−H single bonds, 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 O O H H H + −
Octets first

Draw H₃PO₄ with octets first in the Lewis Structure Generator

Shape and polarity of H₃PO₄

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 H₃PO₄