In a molecule, the number after the element symbol indicates the number of atoms of that element in the molecule. The chemical formula for water is H2O. The two after the H (hydrogen) which stands for hydrogen, indicates that there are two hydrogen atoms in the molecule. Since there are no numbers after the O (Oxygen), that means there is only a single atom of that element.
So, there are two hydrogen atoms and one oxygen atom in one molecule of water.
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