The chemical formula C5H10O makes up the isomers valeraldehyde, pentan-2-one and pentan-3-one. Valeraldehyde is an aldehyde due to the hydrogen attached directly to the carbonyl functional group. Pentan-2-one and pentan-3-one are both ketones due to the attachment of a carbon chain to each end of the molecule's carbonyl.
An isomer is a group of molecules that are all composed of the same chemical formula; however, in each isomer the atoms are arranged differently in geometrical space. The chemical formula C5H10O has many different isomers which include aldehydes, ketones, and esters.
Valeraldehyde is the only aldehyde isomer of C5H10O due to the requirement of the carbonyl functional group being attached to a carbon chain and a single hydrogen. Therefore, the five carbons must be attached in a single chain with the carbonyl at the end. If the carbonyl was present at the opposite end of the carbon chain, it would not be a different molecule. It would instead still be considered valeraldehyde, but with a different spatial arrangement.
A ketone must contain a carbonyl group with carbon chains on both ends. In the case of C5H10O, the carbonyl can be attached to the second or third carbon on the molecule to meet this requirement. These isomers are respectively named pentan-2-one and pentan-3-one.