Isomers are defined as

• A. Different atoms, different arrangements
• B. Same atoms, same arrangements
• C. Same atoms, different arrangements
• D. Different atoms, same arrangements

Answer: (C)

Isomers are compounds that share the same molecular formula but differ in how the atoms are arranged. The best choice captures this by saying the same atoms are present, but they are arranged differently, either in how they are connected (structural isomers) or in how they are oriented in space (stereoisomers). This distinction explains why two compounds can have identical formulas yet be different substances.

For example, butane and isobutane both have the formula C4H10, but their carbon skeletons are connected differently, giving them different properties. Likewise, glucose and fructose have the same atoms in the same overall formula but differ in how those atoms are arranged, resulting in different structures and functions.

If the atoms were different, the formula would change, so they wouldn’t be isomers. If the arrangement were the same, you’d have the same molecule, not an isomer.