What is functional group isomerism? Explain with a suitable example.

When two or more compounds have the same molecular formula but possess different functional groups,they are called functional isomers,and this phenomenon is known as functional group isomerism.
Example: The molecular formula $C_3H_6O$ represents both an aldehyde and a ketone.
$1$. Propanal $(CH_3CH_2CHO)$: Contains an aldehyde functional group $(-CHO)$.
$2$. Propanone $(CH_3COCH_3)$: Contains a ketone functional group $(>C=O)$.