What defines isomers in chemistry?

Isomers are defined as compounds that share the same molecular formula but possess different structural arrangements. This means they contain the same types and numbers of atoms, yet the way those atoms are connected or arranged in space is distinct, leading to variations in their chemical and physical properties. The different arrangements can result in isomers that differ in boiling points, solubility, and reactivity, among others.

Understanding isomerism is fundamental in chemistry because even small changes in structure can lead to significant differences in behavior. This concept also plays a crucial role in fields such as pharmacology, where the activity of a drug can be altered by its structural isomerism.

The other options describe concepts that do not align with the definition of isomers. The idea of compounds having different molecular formulas pertains to entirely different compounds rather than isomers. Similarly, compounds that have identical properties or react in the same manner would not be classified as isomers, since those would imply a lack of structural variation despite having the same formula.