The benzene ring consists of six sp2 hybridized carbon atoms in a regular hexagon. Each carbon forms two σ-bonds with adjacent carbons, and a third σ-bond with a hydrogen atom. The 2p orbital on each atom is available for π-bonding.
Interaction between the p orbitals on the six carbon atoms forms a conjugated system of π-electrons. Molecules with this bonding arrangement are called aromatic molecules.
For convenience, we usually draw benzene as one of its resonance forms. The shape of benzene
Benzene is a planar regular hexagon, with bond angles of 120°. This is easily explained. It is a regular hexagon because all the bonds are identical.
The energetic stability of benzene
This is accounted for by the delocalisation. As a general principle, the more you can spread electrons around - in other words, the more they are delocalised - the more stable the molecule becomes. The extra stability of benzene is often referred to as "delocalisation energy".
The reluctance of benzene to undergo addition reactions
With the delocalised electrons in place, benzene is about 150 kJ mol-1 more stable than it would otherwise be. If you added other atoms to a benzene ring you would have to use some of the delocalised electrons to join the new atoms to the ring. That would disrupt the delocalisation and the system would become less stable.
Since about 150 kJ per mole of benzene would have to be supplied to break up the delocalisation, this isn't going to be an easy thing to do.
The symbol for benzene
Although you will still come across the Kekulé structure for benzene, for most purposes we use the structure on the right.
The hexagon shows the ring of six carbon atoms, each of which has one hydrogen attached. (You have to know that - counting bonds to find out how many hydrogens to add doesn't work in this particular case.)
The circle represents the delocalised