Earth's Minerals (Book)

A molecule is the smallest unit of a chemical compound. A compound is a substance made of two or more elements. The elements in a chemical compound are always present in a certain ratio. 

Water is probably one of the simplest compounds that you know. A water molecule is made of two hydrogen atoms and one oxygen atom (Figure). All water molecules have the same ratio: two hydrogen atoms to one oxygen atom.

A water molecule has two hydrogen atoms (shown in gray) bonded to one oxygen molecule (shown in red).

A water molecule has two hydrogen atoms (shown in gray) bonded to one oxygen molecule (shown in red).