Boron trifluoride (BF₃) Lewis structure
24 valence electrons, trigonal planar
Build BF₃ step by step in the Lewis Structure Generator
What is the Lewis structure of BF₃?
Boron trifluoride, BF₃, has 24 valence electrons. Boron is the central atom, joined by three B−F single bonds. It carries three lone pairs on each F. Every formal charge is zero. B is AX₃: trigonal planar, with an ideal bond angle of 120°, and the molecule is nonpolar.
Valence electrons in BF₃
In one line: 3 + 3 × 7 = 24. Each atom brings the valence electrons its group gives it.
How to draw the BF₃ Lewis structure
- Count the valence electrons. Valence electrons: B 3 and each F 7. 3 + 3 × 7 = 24.
- Choose the central atom. B goes in the middle: B is the least electronegative atom other than H, χ 2.04 against 3.98 for F.
- Join the atoms with single bonds. Three single bonds use 6 electrons, leaving 24 − 6 = 18.
- Complete the outer atoms. Each outer atom takes enough lone pairs to reach eight: 3 F × 6 = 18, leaving 18 − 18 = 0.
- Put the rest on the central atom. Nothing is left, so B gets no lone pair, and B now has 6 electrons round it.
- Make multiple bonds where needed. B is left with 6 electrons. A double bond from an outer atom would complete its octet only by putting positive charge on the more electronegative atom, so B stays short, one of the known exceptions to the octet rule.
Lone pairs and formal charges in BF₃
| Atom | Number | Bonds | Lone pairs | Electrons round it | Formal charge, V − N − B/2 |
|---|---|---|---|---|---|
| B (central) | 1 | three single bonds | 0 | 6 | 3 − 0 − 3 = 0 |
| F | 3 | 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 0, the molecule’s overall charge.
Shape and polarity of BF₃
| Central atom | B |
|---|---|
| VSEPR class | AX₃ |
| Electron geometry | trigonal planar |
| Molecular geometry | trigonal planar |
| Ideal bond angle | 120° |
| Measured bond angle | 120° |
| Polarity, an estimate | nonpolar |
In a trigonal planar shape identical bonds cancel exactly.
B has three σ bonds and no lone pairs, three electron domains in all, so its electron geometry is trigonal planar. With no lone pairs, the molecular geometry is the same.
The VSEPR Molecular Geometry viewer turns the AX₃ shape in 3D, lone pairs included.
The measured angle is the accepted experimental value the VSEPR viewer quotes for this molecule.
Notes on the BF₃ Lewis structure
- Boron stays at six electrons. A B=F double bond would complete its octet only by putting +1 on fluorine, the most electronegative element, which no convention prefers.
Work with BF₃
- Lewis Structure Generator: BF₃ 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