The single Lewis structure based on the presence of two single bonds and one double bond is not enough to represent the molecule precisely as it presents unequal bonds. Hence, the concept of resonance was introduced to deal with this difficulty.

a) There are three possible resonance structures for SO₃ which are given below:

The structure I, II and III represent the three canonical forms (resonating structures) of SO₃.

The structure IV represents the structure of SO₃ more accurately. This is also called resonance hybrid.

b) There are two possible resonance structures for SO₃ which are given below:

The structure I and II represent the two canonical forms (resonating structures) of NO₂

The structure III represents the structure of NO₂ more accurately. This is also called resonance hybrid.

c) There are three possible resonance structures for NO₃^- which are given below:

The structure I,II and III represent the three canonical forms (resonating structures) of NO₃^-

The structure IV represents the structure of NO₃^- more accurately. This is also called resonance hybrid.