Question

How many resonance structures can be drawn for ozone?

Answer 1

Ozone, or `O_3`, has two major resonance structures that contribute equally to the overall hybrid structure of the molecule.

The total number of valence electrons the ozone molecule has is equal to 18 - 6 electrons from each oxygen atom. The two possible Lewis structures that can be drawn for ozone are

Both structures account for the needed 18 valence electrons - 6 from 3 bonds and 12 as lone pairs placed on the oxygen atoms. The two structures are equivalent from the stability staindpoint, each having a positive and a negative formal charge placed on two of the oxygen atoms.

As a result, both structures will contribute equally to the overall hybrid structure of the molecule, which can be drawn like this

The hybrid structure, shown above on the right, will have two (-1/2) partial negative charges on two of the oxygen atoms and a positive (+1) charge on the third one.

The molecule's `"1/2"` bond order, which represents the number of bonds between a pair of atoms, also supports the idea that the hybrid structure combines the two major resonance structures the ozone molecule has. As a result, the double bond between the three oxygen atoms will have significant single bond character.

Other resonance structures can be drawn for ozone; however, none of them will be major contributors to the hybrid structure. Here are two more possible resonance structures

These structures will be very minor contributors because, most importantly, both have an oxygen atom that lacks a full octet, and because there are fewer covalent bonds present compared with the other two structures, another factor that significantly decreases structure stability.

So, as a conclusion, ozone has two resonance structures that are major contributors to its hybrid structure, and at least two more that are very minor contributors.