I am not clear on what a molecule is. If water is a molecule, is it also a compound because the hydrogen and oxygen have been chemically combined? If so, how do you determine whether a substance is a compound or a molecule?
A molecule is what you get when any atoms join together.
A compound is what you get when atoms of two or more different elements join together.
All compounds are molecules, but not all molecules are compounds.
Water is a molecule because it is made from atoms that have been chemically combined. It is also a compound because the atoms that make water are not all the same - some are oxygen and some are hydrogen.
Oxygen in the atmosphere is a molecule because it is made from two atoms of oxygen. It is not a compound because it is made from atoms of only one element - oxygen. This type of molecule is called a diatomic molecule, a molecule made from two atoms of the same type.
Steve Gagnon, Science Education Specialist (Other answers by Steve Gagnon)