Skip to content
ScienceQuest

Triiodide ion (I₃⁻) Lewis structure

22 valence electrons, linear, a charge of −1

I I I − −
I₃⁻ as the Lewis Structure Generator draws it: lone pairs as dots, nonzero formal charges circled.

Build I₃⁻ step by step in the Lewis Structure Generator

What is the Lewis structure of I₃⁻?

Triiodide ion, I₃⁻, has 22 valence electrons. Iodine is the central atom, joined by two I−I single bonds. It carries three lone pairs on the central I and three on each outer I. Its only formal charge is −1, on the central I. I is AX₂E₃: linear, with an ideal bond angle of 180°.

Valence electrons in I₃⁻

Element Group Valence electrons Atoms Electrons
Iodine 17 7 3 21
Charge of 1− +1
Total 22

In one line: 3 × 7 + 1 = 22. Each atom brings the valence electrons its group gives it, and the negative charge adds one electron for each unit.

How to draw the I₃⁻ Lewis structure

  1. Count the valence electrons. Valence electrons: each I 7, and the 1− charge adds 1. 3 × 7 + 1 = 22.
  2. Choose the central atom. Every atom here is I, so one I goes in the middle and the others bond to it.
  3. Join the atoms with single bonds. Two single bonds use 4 electrons, leaving 22 − 4 = 18.
  4. Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight: 2 I × 6 = 12, leaving 18 − 12 = 6.
  5. Put the rest on the central atom. The 6 left go on the central atom as three lone pairs on I, and I now has 10 electrons round it. That is more than eight, which an atom from period 3 on can hold.
  6. Make multiple bonds where needed. Every atom already has an octet or more, so no lone pair needs to become a bond.

Lone pairs and formal charges in I₃⁻

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
I (central) 1 two single bonds 3 10 7 − 6 − 2 = −1
I 2 one single bond 3 8 7 − 6 − 1 = 0

V is the atom’s valence electrons, N its nonbonding electrons and B its bonding electrons. The formal charges add up to −1, the charge on the ion.

Shape and polarity of I₃⁻

Central atom I
VSEPR class AX₂E₃
Electron geometry trigonal bipyramidal
Molecular geometry linear
Ideal bond angle 180°
Polarity, an estimate not applicable

An ion carries a net charge, so whether its bond dipoles cancel is not the question polarity asks.

I has two σ bonds and three lone pairs, five electron domains in all, so its electron geometry is trigonal bipyramidal. Counting only the bonded atoms, the molecular geometry is linear.

The VSEPR Molecular Geometry viewer turns the AX₂E₃ shape in 3D, lone pairs included.

Work with I₃⁻