Question

Explain why aldehyde and ketone with same number of $C$ atoms are functional isomers.

Answer 1

The aldehyde and ketone with the same number of $C$ atoms are functional isomers because the aldehyde and ketone with the same number of $C$ atoms have same molecular formulas but differ in their functional groups.

For example, as shown in the above image they are functional isomers because they have the same molecular formula but differ in their functional groups.