Isomers are molecules or polyatomic ions with identical molecular formula but different chemical structures or arrangements of atoms. For example, glucose and fructose are isomers of each other, as they have the same molecular formula (C6H12O6) but different arrangements of atoms. Another example is butane and isobutane, which have the same molecular formula (C4H10) but different structures due to the branching of the carbon chain. Isomers can have different physical and chemical properties, such as boiling point, melting point, and reactivity, depending on their structures.