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ScienceQuest

Perchloric acid (HClO₄) Lewis structure

32 valence electrons, tetrahedral

Cl O O O O H
HClO₄ as the Lewis Structure Generator draws it: lone pairs as dots.

Build HClO₄ step by step in the Lewis Structure Generator

What is the Lewis structure of HClO₄?

Perchloric acid, HClO₄, has 32 valence electrons. Chlorine is the central atom, joined by three Cl=O double bonds, one Cl−O single bond and one O−H single bond. It carries two lone pairs on the O of the O−H and two on each double-bonded O. Every formal charge is zero. Cl is AX₄: tetrahedral, and the molecule is polar.

Valence electrons in HClO₄

Element Group Valence electrons Atoms Electrons
Hydrogen 1 1 1 1
Chlorine 17 7 1 7
Oxygen 16 6 4 24
Total 32

In one line: 1 + 7 + 4 × 6 = 32. Each atom brings the valence electrons its group gives it.

How to draw the HClO₄ Lewis structure

  1. Count the valence electrons. Valence electrons: H 1, Cl 7 and each O 6. 1 + 7 + 4 × 6 = 32.
  2. Choose the central atom. Cl goes in the middle: Cl is the least electronegative atom other than H, χ 3.16 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. Five single bonds use 10 electrons, leaving 32 − 10 = 22.
  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 + 3 O × 6 = 22, leaving 22 − 22 = 0.
  5. Put the rest on the central atom. Nothing is left, so Cl gets no lone pair, and Cl now has 8 electrons round it.
  6. Make multiple bonds where needed. Cl already has an octet, but turning three lone pairs into three Cl=O bonds lowers its formal charge from +3 to 0. That is what the smallest formal charges convention asks for; octets first stops a step earlier.

Lone pairs and formal charges in HClO₄

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
Cl (central) 1 three double bonds and one single bond 0 14 7 − 0 − 7 = 0
O 1 two single bonds 2 8 6 − 4 − 2 = 0
O 3 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.

HClO₄ with an octet on every atom

The structure above keeps the formal charges as small as they go and puts 14 electrons round Cl. Keeping every atom to eight instead, the octets-first convention, gives one form: four Cl−O single bonds and one O−H single bond, with formal charges of +3 on the central Cl and −1 on each single-bonded O. Courses differ on which of the two they expect.

Cl O O O O H 3+ − − −
Octets first

Octets first puts +3 on chlorine. The smallest formal charges convention avoids it with expanded octets.

Draw HClO₄ with octets first in the Lewis Structure Generator

Shape and polarity of HClO₄

Central atom Cl
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.

Cl 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 HClO₄