What defines a compound?

A compound is defined as a substance that consists of two or more different elements that are chemically bonded together in a specific and fixed ratio. This means that the particular proportions of these elements are not random; they follow a definite pattern that corresponds to the chemical formula of the compound. For example, water (H₂O) is a compound made up of two hydrogen atoms and one oxygen atom, demonstrating a consistent and repeating ratio.

In contrast, the other responses describe different types of matter. A single type of atom refers to an element, not a compound, as compounds require multiple types of elements. A mixture of elements means that the elements can be physically combined but are not chemically bonded, which distinguishes them from compounds. Lastly, referring to an element with varying size does not pertain to the definition of a compound; elements themselves can vary in size and atomic mass, but this does not affect their classification as compounds or otherwise. Therefore, the defining characteristic of compounds is specifically their formation from different elements bonded in a fixed composition.