Why is boron trifluoride a planar molecule?

Boron trifluoride is a planar molecule because it has a trigonal planar molecular geometry due to its sp2 hybridisation.

Boron trifluoride, or BF3, is a molecule that consists of one boron atom and three fluorine atoms. The boron atom is in the centre of the molecule, with the three fluorine atoms surrounding it. The reason why this molecule is planar, or flat, is due to its molecular geometry and the type of hybridisation it undergoes.

The boron atom in BF3 has three valence electrons, and it forms three sigma bonds with the three fluorine atoms. This leaves the boron atom with no lone pairs of electrons. The absence of lone pairs on the central atom and the presence of only three regions of electron density around the boron atom result in a trigonal planar molecular geometry. This geometry is characterised by bond angles of 120 degrees and all the atoms lying in the same plane, hence making the molecule planar.

Furthermore, the boron atom in BF3 undergoes sp2 hybridisation. Hybridisation is a process in which atomic orbitals mix to form new, identical hybrid orbitals. In the case of BF3, one s orbital and two p orbitals in the boron atom hybridise to form three sp2 hybrid orbitals. These hybrid orbitals are arranged in a trigonal planar shape, further contributing to the planar structure of the molecule.

In summary, the planar structure of boron trifluoride is a result of its trigonal planar molecular geometry and sp2 hybridisation. The absence of lone pairs on the central boron atom and the presence of only three regions of electron density lead to this geometry. The sp2 hybridisation of the boron atom aligns the hybrid orbitals in a planar arrangement, reinforcing the planar structure of the molecule.