Positional isomerism refers to a type of structural isomerism in organic chemistry where compounds have the same molecular formula but differ in the position of functional groups or substituents on the carbon skeleton. In other words, positional isomers have the same atoms but arranged differently.

Example:
Let's consider the compound C4H9Br, which can exhibit positional isomerism. In this case, the bromine atom (Br) can be attached to different carbon atoms in the carbon chain, resulting in different positional isomers.

1. 1-Bromobutane: In this isomer, the bromine atom is attached to the first carbon atom in the butane chain.

2. 2-Bromobutane: Here, the bromine atom is attached to the second carbon atom in the butane chain.

3. 3-Bromobutane: In this isomer, the bromine atom is attached to the third carbon atom in the butane chain.

4. 4-Bromobutane: In this isomer, the bromine atom is attached to the fourth carbon atom in the butane chain.

Explanation:
Positional isomerism arises due to the different arrangements of substituents or functional groups on a carbon chain. The position of attachment of these groups can greatly influence the physical and chemical properties of a compound.

Key Points:
- Positional isomers have the same molecular formula but different structural arrangements.
- The position of functional groups or substituents on the carbon chain determines the positional isomerism.
- Positional isomers exhibit different physical and chemical properties due to variations in the arrangement of atoms.
- The systematic naming of positional isomers is based on the numbering of the carbon atoms in the chain.
- Positional isomerism is commonly observed in organic compounds with multiple carbon atoms and functional groups.

Conclusion:
Positional isomerism is a type of structural isomerism where compounds have the same molecular formula but differ in the position of functional groups or substituents on the carbon skeleton. Understanding positional isomerism is important in organic chemistry as it helps in predicting and explaining the different properties and behaviors of compounds with similar molecular formulas.