Isomers differ primarily in what aspect?

Isomers are compounds that share the same molecular formula but differ in their structural arrangement of atoms. This difference in the way atoms are connected or arranged leads to variations in their structural formulas, which is the defining characteristic of isomers. For example, butane (C4H10) has two structural isomers: n-butane, with a straight-chain configuration, and isobutane, which has a branched structure.

While isomers can also exhibit differences in chemical properties, molecular weight, and solubility, these differences arise from the variation in their structural formulas. Therefore, the fundamental aspect that sets isomers apart from one another is indeed the structural formula, as it directly influences their chemical and physical properties.