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Formulas tell what makes up a compound. They show a chemical reaction and are written like a math equation. The smallest particle of a compound would be a molecule of that compound.
One of the best known formulas is the formula for water. Water is made up of two different elements, hydrogen and oxygen. One molecule of water is made up of two atoms of hydrogen and one atom of oxygen. The symbol for hydrogen is H and the symbol for oxygen is O. The formula for water would be H2O. If there is only one atom in the formula, no number is affixed to the symbol.
Some formulas are simple and some are very complex.
Some formulas can be stated several ways. The molecular version tells the exact composition of a molecule of the compound. An empiracal formula gives a simpler version that is rather like a reduced fraction. For example:
Molecular formula for glucose: C6H12O6
Empiracal formula for glucose: CH2O