In a given molecule, the arrangement of atoms around a central atom in 3D space represents the shape or the geometry of the molecule. These atoms are linked together by chemical bonds formed by the participation of the valence electrons.
In general, a pair of electrons are involved in the formation of a single bond, these are known as bond pairs. In addition, electrons that are not involved in bonding are known as lone pairs. It is the number of bond pairs and lone pairs of electrons around the central atom that helps in the prediction of the shape of a molecule.
A trigonal planar molecule is such that the central atom is bonded to 3 other atoms in which the bond angles are 120°. Since each bond represents a pair of electrons there will be 3 electron domains around the central atom. For example, BF₃ has a trigonal planar structure ( 3 bond pairs + 0 lone pairs).
