Q.In the following resonating structures A and B, the number of unshared electrons in valence shell present on oxygen respectively are

Step 1. Identify the two structures. Section 12.7 (Fig. 12.4) draws exactly this pair for a generic carbonyl group: (A) the neutral, major resonance contributor, a plain C=O double bond; (B) a minor, charge-separated resonance contributor, in which the pi electrons have shifted fully onto oxygen, leaving a single C-O bond with a formal negative charge on oxygen and a formal positive charge on carbon.

Step 2. Count oxygen's unshared (lone-pair) electrons in A. In the neutral form, oxygen is doubly bonded to carbon and carries 2 lone pairs -- so 2 x 2 = 4 unshared electrons.

Step 3. Count oxygen's unshared electrons in B. In the charge-separated form, oxygen is singly bonded to carbon and carries the extra negative charge, meaning it now holds 3 lone pairs -- so 3 x 2 = 6 unshared electrons.

Step 4. Match to the options. A gives 4, B gives 6, so the pair is (4, 6).

Recognising A and B as this chapter's own Fig. 12.4 resonance pair for the carbonyl group, then counting oxygen's lone pairs (unshared electrons) in the neutral double-bonded form versus the charge-separated single-bonded form

• Counting the BONDING electrons (the shared pair(s) in the C-O bond) instead of only the UNSHARED (lone-pair) electrons on oxygen.
• Assuming the neutral, double-bonded form (A) has MORE unshared electrons than the charge-separated form (B), when in fact moving a bonding pair onto oxygen as a lone pair in B increases its unshared-electron count from 4 up to 6.