Trigonal planar geometry

What is trigonal planar geometry?

The definite relative arrangement of the bonded atom in a covalent molecule is known as the geometry of the molecule. A trigonal planar compound, it has a central atom attached to three atoms arranged in a triangular shape around the central atom. All four atoms lie flat on a plane.

In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°.

Table of contents

• Structure of trigonal planar geometry
• Feature of trigonal planar geometry
• Examples of trigonal planar geometry compound
• FAQs

Structure of trigonal planar geometry

Feature of trigonal planar geometry

• This molecule is made up of 3 equally spaced sp² hybrid orbitals arranged at 120⁰ angles.
• The shape of the orbitals is planar triangular.
• Since there is an atom at the end of each orbital, the geometry of the molecule is also trigonal planar.
• In this geometry the general formula is AX₃. Here all the four atoms lie in one plane.
• The point group of trigonal planar geometry is D_3h.
• Example BF₃, BCl₃, AlCl₃, SO₃, AlF₃ etc.

Examples of trigonal planar geometry compound

(a) Boron trichloride molecule (BCl₃)

• In this molecule boron is the central atom.
• The atomic number of boron atom is 5 and its electron configuration 1s² 2s² 2p¹ .
• The valence electron of boron atom is 3.
• The three valence electrons from three shared pairs bond with the unpaired electron of the three chlorine atoms.
• The hybridisation of the molecule is sp² hybridisation.
• Due to the presence of three shared pair electrons, the geometry of the molecule is regular. Since the repulsive interaction in the electron pairs has to be minimum, therefore the shape of the BCl₃ molecule is trigonal planar.
• The bond angle is 120⁰.

(b) Sulphur trioxide molecule (SO₃)

• Sulphur trioxide has three bonding positions with partial double bond character in each.
• The best positions for the oxygens in this molecule are at the comers of an equilateral triangle.
• The bond angle of O – S – O bond is 120⁰
• The multiple bonding does not affect the geometry because it is shared equally among the three bonds.