What does isomerism refer to in organic chemistry?

Isomerism in organic chemistry specifically refers to the phenomenon where compounds have the same molecular formula but differ in their structural arrangement or connectivity of atoms. These variations lead to distinct chemical properties and behaviors despite having the same types and numbers of atoms. For example, butane (C4H10) can exist as either n-butane or isobutane, showcasing how the structural arrangement significantly impacts physical and chemical properties.

The other options describe different chemical concepts: the loss of electrons during a reaction relates to oxidation-reduction processes, existing compounds with the same structure wouldn't qualify as isomers since they cannot be distinct entities, and the rearrangement of atoms is more aligned with reaction mechanisms rather than the definition of isomerism itself. Overall, the essence of isomerism lies in the diversity of arrangements that yield compounds with identical molecular formulas but unique structural identities.