Why is boron trifluoride considered planar?

Boron trifluoride is considered planar 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 molecular geometry of boron trifluoride is determined by the arrangement of these atoms and the electron pairs around the central boron atom.

The boron atom in BF3 has three valence electrons, and each fluorine atom has seven. When the boron atom forms covalent bonds with the three fluorine atoms, it shares one of its electrons with each fluorine atom. This results in the boron atom having a full outer shell of electrons, and each fluorine atom also having a full outer shell.

The electron pairs in the boron trifluoride molecule are arranged in a way that minimises the repulsion between them. This is known as the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to this theory, the electron pairs will arrange themselves in a trigonal planar shape, which is a flat, three-sided shape. This is the most stable arrangement for three electron pairs, and it results in the molecule having a planar shape.

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

Therefore, due to its sp2 hybridisation and the arrangement of its electron pairs according to the VSEPR theory, boron trifluoride is considered planar.