CgHg, inside

Benzene is sometimes represented by its chemical formula CgHg and sometimes (to show structure) by a hexagon with a circle inside it ... 

A computer-generated model of a molecule of benzene, CgHg. A ball-and-stick model is shown inside a representation of the molecular surface. 

A special class of cyclic unsaturated hydrocarbons is known as the aromatic hydrocarbons. The simplest of these is benzene (CgHg), which has a planar ring structure, as shown in Fig. 22.11(a). In the localized electron model of the bonding in benzene, resonance structures of the type shown in Fig. 22.11(b) are used to account for the known equivalence of all the carbon-carbon bonds. But as we discussed in Section 9.5, the best description of the benzene molecule assumes that sp hybrid orbitals on each carbon are used to form the C—C and C—H a bonds, while the remaining 2p orbital on each carbon is used to form tt molecular orbitals. The delocalization of these tt electrons is usually indicated by a circle inside the ring [Fig. 22.11(c)]. 

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