What could I take out/add/replace to make this shorter? A particular atom will have the same number of protons and electrons. Most atoms have at least as many neutrons as protons. An element is a substance that is made entirely from one type of atom. A compound is a substance made from two or more different elements that have been chemically joined. Water's extensive capability to dissolve a variety of molecules has earned it the designation of universal solvent, and it is this ability that makes water such an invaluable life-sustaining force. On a biological level, water's role as a solvent helps cells transport and use substances like oxygen or nutrients. The 4 main categories of biological (organic) molecules are carbohydrates, nucleic acids, proteins, and lipids. All four of those types of molecules contain carbon, hydrogen, and oxygen.