Chemical formulas: counts of atoms
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Compounds are represented using formulas. These use the element symbols and subscripts. The element symbols tell what kinds of atoms are in the compound. The subscripts tell the number of each type of atom. Natural gas is mainly methane which has the formula CH4. There are four hydrogen atoms and one carbon atom in a methane molecule. The subscripts are essential so the count of atoms of each element are correct for the compound. If the subscripts are changed then the formula represents a different substance. Increased amounts of a substance mean more of the formula units are present. 
Other model examples of formulas and structures for molecules are shown here.
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