How is a compound defined?

A compound is defined as a substance made of joined atoms of two or more different elements. This definition highlights the key characteristic of a compound: it consists of distinct elements that chemically bond together, resulting in a substance with unique properties different from those of the individual elements. For example, water (H₂O) is a compound formed from hydrogen and oxygen atoms bonded together. This definition emphasizes the chemical interaction and stability that occurs when elements combine, which is fundamental in chemistry.

In contrast, the other options refer to different concepts. A single element in isolation does not involve any combination with other elements, and thus cannot be classified as a compound. A mixture of different substances refers to physical combinations of elements or compounds that retain their individual properties, rather than a chemical bonding seen in compounds. Lastly, an atom with varying isotopes pertains to atoms of a single element that have different numbers of neutrons, which does not relate to the concept of a compound formed by multiple elements.