29.3 Shapes of Aromatic Organic Molecules: σ and π Bonds

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Describe and explain the shape of benzene and other aromatic molecules, including sp² hybridisation, in terms of σ bonds and a delocalised π system.

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What is an Aromatic Molecule?

An aromatic molecule is a molecule that contains at least one ring of carbon atoms with a special type of electron arrangement called a conjugated π (pi) system. The word "conjugated" simply means that there are alternating single and double bonds around the ring — and because of this arrangement, the electrons don't stay fixed between just two atoms. Instead, they spread out across the whole ring. This spreading-out of electrons is called delocalisation.

The most important and well-known aromatic molecule is benzene (molecular formula C₆H₆). It is made up of six carbon atoms arranged in a flat, regular hexagon, with one hydrogen atom attached to each carbon.

The Shape of Benzene

Benzene is:

  • Flat (planar) — all six carbon atoms and all six hydrogen atoms lie in exactly the same flat plane, like a drawing on a piece of paper.
  • Regular — all six carbon–carbon bonds are exactly the same length. This is very important and we will explain why shortly.
  • Bond angles of 120° — the angle between any two bonds around each carbon atom is 120°. This gives benzene its perfect hexagonal shape.

sp² Hybridisation — What Does This Mean?

To understand why benzene is flat with 120° bond angles, you need to understand something called sp² hybridisation (said as "s-p-two hybridisation").

A Quick Reminder About Orbitals

An orbital is a region of space around an atom's nucleus where electrons are most likely to be found. There are different types of orbitals:

  • s orbitals — spherical (round) in shape
  • p orbitals — shaped like a dumbbell (two lobes pointing in opposite directions)

A carbon atom in its ground state (its normal, lowest-energy state) has one s orbital and three p orbitals available in its outer shell.

What Happens During sp² Hybridisation?

When carbon atoms form part of a benzene ring, something special happens to their orbitals. One s orbital mixes with two of the three p orbitals to create three new, identical orbitals called sp² hybrid orbitals.

Think of it like mixing three different colours of paint together — you end up with three pots of a new, identical colour.

The key points about sp² hybridisation are:

  • Three sp² orbitals are formed — these point outwards from the carbon atom, spread equally at 120° angles to each other, all in the same flat plane. This is why the bond angles in benzene are 120° and why benzene is flat.
  • One p orbital is left over (unhybridised) — this leftover p orbital sits above and below the flat plane of the ring, like a dumbbell sticking straight up and down. This leftover p orbital is very important — it is responsible for the π system.

Every single carbon atom in benzene undergoes sp² hybridisation. So there are six carbon atoms, each with three sp² orbitals and one leftover p orbital.

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