What is a compound?

A compound is defined as a substance formed when two or more different elements chemically combine in a fixed ratio. This means that the elements undergo a chemical reaction that results in the formation of a new substance with distinct properties that are different from the individual elements that make it up. For example, water (H₂O) is a compound made from hydrogen and oxygen. The chemical bonding between these elements results in a new substance that has unique characteristics, such as being a liquid at room temperature.

In contrast, other options describe different concepts. A mixture of elements refers to when elements are combined physically but not chemically, meaning they retain their individual properties. A single type of element refers to pure substances composed of only one kind of atom, such as oxygen (O₂) or gold (Au). Lastly, atoms that vary in number describes isotopes or just the nature of atoms themselves without indicating a chemical combination, thereby not fitting the definition of a compound.