Compounds are made up of two or more elements that have chemically bonded together. The atoms of different elements are held together by chemical bonds, which form when electrons are shared or exchanged between atoms. Compounds can be composed of two or more non-metallic elements, or of a metal and a non-metal. In any case, the elements that make up a compound are always present in a fixed proportion by mass. For example, water is a compound made up of hydrogen and oxygen, and it always contains two atoms of hydrogen for every one atom of oxygen.