A compound is formed when two or more different elements chemically combine in a fixed ratio. For example, water (H₂O) is a compound made of two hydrogen atoms and one oxygen atom.

It's important to note that while a compound must have at least two different elements, it can have multiple atoms of each element. For instance, glucose (C₆H₁₂O₆) has six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.