What are compounds called that have the same number of atoms of the same elements but different structures?

The correct answer is "isomers." Isomers are compounds that share the same molecular formula, meaning they are composed of the same number of atoms of the same elements, yet differ in the arrangement of those atoms, leading to different structural properties. This structural variation can significantly influence their physical and chemical properties.

In contrast, allotropes refer specifically to different forms of the same element in the same physical state, such as carbon existing as graphite or diamond. Homologs typically describe compounds that share a similar structure but differ by a repeating unit, often seen in organic compounds with varying chain lengths. Complexes refer to compounds formed from metal ions bound to molecules or ions, which often do not fit the criteria of having the same molecular formula and distinct structures.

Thus, isomers encompass a broader range of structural diversity based on identical molecular formulas, making this term the most accurate choice for the given question.