Resonance in  Benzene

Benzene (C₆H₆) is a ring structure that is common in organic compounds known as aromatics. Here, there are electrons in an orbital that covers 6 nuclei, and there are two ways of drawing the Lewis dot structures of benzene.

Fig. 1.1: Lewis dot structure of benzene. 

Note here we have 3 single and 3 double bonds in the ring. In organic chemistry the carbon and hydrogen atoms are not explicitly drawn as in figure 1.1, but implicitly inferred, as in figure 1.2. This is done by assuming each line ends with a carbon, and each carbon has 8 electrons around it, and so has 4 bonds.

Fig. 1.2: This is the same as figure 1.1, except that the carbons and hydrogens are implied.

As each structure has 6 corners with a carbon at each end, there are 6 carbons. As each corner is the junction of a single and a double bond, there are 6 electrons around each carbon, and so we assume there is one additional bond with the remaining two electrons being shared with hydrogen.

It is very common to merge the two electrons into one structure, which is often done with a solid ring. 

Fig. 1.3: Condensed drawings of benzene resonance structures into a single Lewis dot structure. 

Note here, that the second bond represented by the circles represents an orbital that covers 6 nuclei.