Functional Isomerism Explanation:
Functional isomerism is a type of structural isomerism where compounds have the same molecular formula but differ in the functional groups present. This type of isomerism arises due to the different ways in which atoms can be arranged within a molecule to form different functional groups.

Examples of Functional Isomerism:
- Aldehydes and Ketones: Isomers that have the same number of carbon and hydrogen atoms but differ in the placement of the carbonyl group. For example, propanal and acetone are functional isomers.
- Alcohols and Ethers: Isomers that have the same number of carbon and hydrogen atoms but differ in the presence of the oxygen atom in the functional group. For example, ethanol and dimethyl ether are functional isomers.

Not Functional Isomerism:
One example of isomerism that is not functional isomerism is chain isomerism. In chain isomerism, compounds have the same molecular formula but differ in the arrangement of the carbon skeleton. This type of isomerism does not involve a change in the functional group present in the molecule.
For example, consider the isomers of pentane: n-pentane, isopentane, and neopentane. These compounds have the same functional group (alkane) but differ in the arrangement of the carbon chain. This is an example of chain isomerism and not functional isomerism.
In conclusion, functional isomerism involves differences in functional groups, while chain isomerism involves differences in the arrangement of the carbon skeleton. It is important to distinguish between these types of isomerism when studying organic chemistry.