Borohydride ion (BH₄⁻) Lewis structure
8 valence electrons, tetrahedral, a charge of −1
Build BH₄⁻ step by step in the Lewis Structure Generator
What is the Lewis structure of BH₄⁻?
Borohydride ion, BH₄⁻, has 8 valence electrons. Boron is the central atom, joined by four B−H single bonds. It carries no lone pairs. Its only formal charge is −1, on the central B. B is AX₄: tetrahedral, with an ideal bond angle of 109.5°.
Valence electrons in BH₄⁻
| Element | Group | Valence electrons | Atoms | Electrons |
|---|---|---|---|---|
| Boron | 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 BH₄⁻ Lewis structure
- Count the valence electrons. Valence electrons: B 3 and each H 1, and the 1− charge adds 1. 3 + 4 × 1 + 1 = 8.
- Choose the central atom. B 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.
- Join the atoms with single bonds. Four single bonds use 8 electrons, leaving 8 − 8 = 0.
- Complete the outer atoms. Every outer atom is H, and its one bond already gives it the two electrons it holds. 0 remain.
- Put the rest on the central atom. Nothing is left, so B gets no lone pair, and B now has 8 electrons round it.
- 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 BH₄⁻
| Atom | Number | Bonds | Lone pairs | Electrons round it | Formal charge, V − N − B/2 |
|---|---|---|---|---|---|
| B (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 BH₄⁻
| Central atom | B |
|---|---|
| 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.
B 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 BH₄⁻
- Lewis Structure Generator: BH₄⁻ built step by step, with every formal charge worked out
- VSEPR Molecular Geometry: every VSEPR shape in 3D, with its bond angles
- All 78 Lewis structures: valence electrons, resonance, shape and polarity in one table