Butanal and butanone have different chemical and physical properties primarily because of differences in their
(1) functional groups
(2) molecular masses
(3) molecular formulas
(4) number of carbon atoms per molecule

(1) functional groups

Explanation:

Functional groups are specific group of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules.

The compounds having similar molecular formula but different arrangement of atoms or groups in space are called isomers and the phenomenon is called as isomerism.  

Butanal is and butanone is .

and are functional isomers which have same molecular formula , similar masses and same number of carbon atoms per molecule but different functional groups attached.