What is a compound?

A compound is a substance that consists of two or more elements that are chemically bonded together in a specific ratio. This definition highlights that compounds have a fixed composition and unique properties that are different from the individual elements that make them up. For instance, water (H₂O) is a compound made of hydrogen and oxygen atoms chemically combined in a 2:1 ratio.

On the other hand, other options describe aspects that do not align with the definition of a compound. A substance that cannot be broken down refers to a pure element or possibly certain compounds in a non-chemical sense but does not explain the chemical nature of a compound. A substance made of only one type of atom describes an element, not a compound. Lastly, a mixture of two different elements refers to a combination where the elements retain their individual properties and are not chemically bonded, which is distinctly different from a compound.