What defines a compound?

A compound is defined as a substance formed when two or more elements are chemically combined in a fixed ratio. This chemical combination involves the formation of chemical bonds, which distinguishes compounds from mixtures. For example, when hydrogen and oxygen chemically combine to form water, they create a compound with distinct properties that are different from those of the individual elements.

The focus on chemical bonding is crucial here; unlike a physical mixture where elements maintain their individual properties, a compound exhibits new characteristics that arise from its unique molecular structure. This is why the definition emphasizing chemical combination is the correct understanding of what constitutes a compound.

In contrast, options describing physical mixtures or referring exclusively to single elements do not capture the essence of compounds, which are fundamentally about chemical interactions, not mere physical proximity or elemental purity.