Look back at the resonance forms of the acetate ion and the acetone anion shown in the previous section. The pattern seen there is a common one that leads to a useful technique for drawing resonance forms. In general, any three-atom grouping with a p orbital on each atom has two resonance forms: The atoms X, Y, and Z in the general structure might be C, N, O, P, S, or others, and the asterisk (*) might mean that the p orbital on atom Z is vacant, that it contains a single electron, or that it contains a lone pair of electrons. The two resonance forms differ simply by an exchange in position of the multiple bond and the asterisk from one end of the three-atom grouping to the other. By learning to recognize such three-atom groupings within larger structures, resonance forms can be systematically generated. Look, for instance, at the anion produced when H + is removed from 2,4-pentanedione by reaction with a base. How many resonance structures does the resultant anion have? The 2,4-pentanedione anion has a lone pair of electrons and a formal negative charge on the central carbon atom, next to a C=O C=O bond on the left. The O=C–C : − O=C–C : − grouping is a typical one for which two resonance structures can be drawn. Just as there is a C=O C=O bond to the left of the lone pair, there is a second C=O C=O bond to the right. Thus, we can draw a total of three resonance structures for the 2,4-pentanedione anion. Worked Example \(\PageIndex{1}\): Drawing Resonance Forms for an Anion Draw three resonance structures for the carbonate ion, CO 3 2 – . Strategy Look for three-atom groupings that contain a multiple bond next to an atom with a p orbital. Then exchange the positions of the multiple bond and the electrons in the p orbital. In the carbonate ion, each singly bonded oxygen atom with three lone pairs and a negative charge is adjacent to the C=O C=O double bond, giving the grouping . Solution Exchanging the position of the double bond and an electron lone pair in each grouping generates three resonance structures. Worked Example \(\PageIndex{2}\): Drawing Resonance Forms for a Radical Draw three resonance forms for the pentadienyl radical, where a radical is a substance that contains a single, unpaired electron in one of its orbitals, denoted by a dot ( · ). Strategy Find the three-atom groupings that contain a multiple bond next to an atom with a p orbital. Solution The unpaired electron is on a carbon atom next to a C=C C=C bond, giving a typical three-atom grouping that has two resonance forms. In the second resonance form, the unpaired electron is next to another double bond, giving another three-atom grouping and leading to another resonance form. Thus, the three resonance forms for the pentadienyl radical are: Exercise \(\PageIndex{1}\) Which of the following pairs of structures represent resonance forms, and which do not? Explain. Answer The structures in (a) are resonance forms. Exercise \(\PageIndex{2}\) Draw the indicated number of resonance forms for each of the following substances: The methyl phosphate anion, CH 3 OPO 3 2 – (3 resonance structures) The nitrate anion, NO 3 – (3) The allyl cation, H 2 C=CH–CH 2 + (2) The benzoate anion (2) Answer