What is the difference between a compound and a molecule?

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A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds.

Molecular hydrogen (H₂), molecular oxygen (O₂) and molecular nitrogen (N₂) are not compounds because each is composed of a single element. Water (H₂O), carbon dioxide (CO₂) and methane (CH₄) are compounds because each is made from more than one element. The smallest bit of each of these substances would be referred to as a molecule. For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen while a single molecule of water is made from two atoms of hydrogen and one atom of oxygen.

What is an element? How many elements are there?

What is a mixture?

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?

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

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