I have read several definitions that basically say a molecule is formed when two or more atoms join together chemically. And a compound is a molecule that contains at least two different types of atoms. All compounds are molecules but not all molecules are compounds. Examples of molecules include H2O, O2, O3 and examples of compounds include NaCl, H2O. Giancoli's text states that "compounds are substances made up of elements, and can be broken down into them; examples include carbon dioxide and water. The smallest piece of an element is an atom; the smallest piece of a compound is a molecule. Molecules are made of atoms." I would interpret Giancoli's definition to say that an H2O molecule by itself is not a compound - but a collection of H2O molecules IS a compound. The same way that a gold atom by itself is not an element, but a collection of gold atoms is an element. Is my interpretation way off?