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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.

Author:

Steve Gagnon, Science Education Specialist (Other answers by Steve Gagnon)

Related Pages:

What is the difference between a compound and a molecule?

What is the simplest way of explaining what an atom, element, compound and mixture are?

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