Understanding Hybridisation of BF₃: Boron Trifluoride

Boron trifluoride (BF₃) is a colourless and toxic gas. It has a sharp odour. It is an important Lewis acid and is widely used in chemical synthesis. Structurally, BF₃ provides one of the clearest examples of sp² hybridisation. It has a central boron (B) atom bonded to three fluorine (F) atoms.

Let’s understand the hybridisation of BF₃ step by step.

What is the Hybridisation of BF₃?

Hybridisation refers to the mixing of atomic orbitals to form new hybrid orbitals of equal energy.

Using the Hybridisation Formula

We can determine the hybridisation using the simple formula:

formula

Step-by-step calculation:

Valence electrons of central atom (Boron) = 3
Number of monovalent atoms (Fluorine atoms) = 3
Negative charge = 0
Positive charge = 0

formula

Interpretation:

A hybridisation number of 3 corresponds to sp² hybridisation.

In sp² hybridisation, 1 s orbital and 2 p orbitals combine to form 3 equivalent sp² hybrid orbitals. These orbitals arrange themselves in a trigonal planar geometry.

Step-by-Step Breakdown of Hybridisation in BF₃

Here is a complete breakdown of boron trifluoride hybridisation:

Electronic Configuration of Boron

The atomic number of Boron (B) = 5

Its ground state configuration:

1s² 2s² 2p¹

In its ground state, boron has only one unpaired electron. It is insufficient to form three bonds.

Therefore, to form 3 bonds, one electron is promoted from 2s to 2p:

So, the excited state configuration will:

2s¹ 2pₓ¹ 2pᵧ¹

Now, there are three unpaired electrons available for bonding.

Ground state vs excited state electronic configuration of boron

Formation of sp² Hybrid Orbitals

Boron undergoes sp² hybridisation to bond with three fluorine atoms:

Orbitals involved: 1 s + 2 p
Result: 3 equivalent sp² hybrid orbitals

These orbitals overlap with the half-filled 2p orbitals of fluorine atoms to form three σ (sigma) bonds.

New configuration:

1s² 2s¹ 2p² → rearranged into three sp² hybrid orbitals

Orbital diagram showing sp² hybridisation in BF₃

Geometry of BF₃

The three sp² orbitals arrange themselves in a planar structure, 120° apart.
According to VSEPR theory, this results in a trigonal planar geometry.
No lone pairs are present on the central boron atom.

Property	Description
Shape	Trigonal Planar
Bond Angle	120°
Bond Type	σ (sp²–p) bonds
Central Atom	Boron
Peripheral Atoms	3 Fluorine atoms
Octet of Boron	Incomplete (6 electrons)
Stability	Stable due to back bonding

BF₃ molecule with trigonal planar shape and 120° bond angles

Back Bonding in BF₃

Boron has an empty 2p orbital.
Fluorine, being highly electronegative, has lone pairs in its 2p orbital.
pπ–pπ back bonding occurs as lone pairs from F are donated to the empty p orbital of B.
Consequently, the bonding takes on a partial double-bond character. This back bonding reduces the bond length and increases the bond order. This partial back bonding stabilises the molecule, even though boron has only 6 electrons in its valence shell.

Formal Charge in BF₃

To verify the stability of BF₃, let’s calculate the formal charges.

Valence electrons in B = 3
Each B–F bond involves 2 electrons (shared)
No lone pairs on B

Formal Charge on B =
= Valence electrons - (Lone pairs + ½ × Bonding electrons)
= 3 - (0 + ½ × 6)
= 3 - 3 = 0

Similarly, each fluorine has a formal charge of 0.
Thus, BF₃ is electrically neutral and optimally stable.

Summing Up

BF₃ is a planar molecule with sp² hybridisation at the central boron atom. The boron atom forms three sigma bonds with fluorine atoms using sp² hybrid orbitals. The molecule has a trigonal planar geometry and a bond angle of 120°. Though electron-deficient, BF₃ is stable due to pπ–pπ back bonding from fluorine to boron.