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After reading this section you will be able to do the following:

  • Explain how chemical formulas can be used to describe elements and molecules.
  • Identify the chemical formulas for several common substances.

What are chemical formulas and how are they used?

In order to make it easier to describe elements and molecules, chemical formulas are used. For example, H represents one atom of hydrogen and "O" represents one atom of oxygen. If we want to represent two atoms of hydrogen, instead of writing H H, we write H2. The subscript "2" means that two atoms of the element hydrogen have joined together to form a molecule. A subscript is only used when more than one atom is being represented. The graphic below illustrates the formula for water using symbols.

Some more common molecules and their chemical formula.

Carbon Dioxide > CO2

Ammonia > NH3

Sugar > C6H12O6

Rubbing Alcohol > C3H7OH


  1. Chemical formulas are used to describe the types of atoms and their numbers in an element or compound.
  2. The atoms of each element are represented by one or two different letters.
  3. When more than one atom of a specific element is found in a molecule, a subscript is used to indicate this in the chemical formula.