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ScienceQuest

Tetrahydroaluminate ion (AlH₄⁻) Lewis structure

8 valence electrons, tetrahedral, a charge of −1

Al H H H H − −
AlH₄⁻ as the Lewis Structure Generator draws it: lone pairs as dots, nonzero formal charges circled.

Build AlH₄⁻ step by step in the Lewis Structure Generator

What is the Lewis structure of AlH₄⁻?

Tetrahydroaluminate ion, AlH₄⁻, has 8 valence electrons. Aluminum is the central atom, joined by four Al−H single bonds. It carries no lone pairs. Its only formal charge is −1, on the central Al. Al is AX₄: tetrahedral, with an ideal bond angle of 109.5°.

Valence electrons in AlH₄⁻

Element Group Valence electrons Atoms Electrons
Aluminum 13 3 1 3
Hydrogen 1 1 4 4
Charge of 1− +1
Total 8

In one line: 3 + 4 × 1 + 1 = 8. Each atom brings the valence electrons its group gives it, and the negative charge adds one electron for each unit.

How to draw the AlH₄⁻ Lewis structure

  1. Count the valence electrons. Valence electrons: Al 3 and each H 1, and the 1− charge adds 1. 3 + 4 × 1 + 1 = 8.
  2. Choose the central atom. Al is the only atom other than H, and H is never in the middle, since it holds only two electrons and so makes one bond.
  3. Join the atoms with single bonds. Four single bonds use 8 electrons, leaving 8 − 8 = 0.
  4. Complete the outer atoms. Every outer atom is H, and its one bond already gives it the two electrons it holds. 0 remain.
  5. Put the rest on the central atom. Nothing is left, so Al gets no lone pair, and Al now has 8 electrons round it.
  6. Make multiple bonds where needed. Every atom already has an octet, so no lone pair needs to become a bond.

Lone pairs and formal charges in AlH₄⁻

Atom Number Bonds Lone pairs Electrons round it Formal charge, V − N − B/2
Al (central) 1 four single bonds 0 8 3 − 0 − 4 = −1
H 4 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 −1, the charge on the ion.

Shape and polarity of AlH₄⁻

Central atom Al
VSEPR class AX₄
Electron geometry tetrahedral
Molecular geometry tetrahedral
Ideal bond angle 109.5°
Polarity, an estimate not applicable

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

Al 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 AlH₄⁻