by Ron Kurtus (revised 27 November 2000)

A chemical formula is similar to an algebraic formula in its use of symbols. Combining different elements forms molecules or compounds. The description of a compound with numbers and symbols is called a chemical formula. Some formulas can be quite complex.

Questions you may have are:

• What are chemical compounds?
• How is a formula different than a compound?
• How do you balance chemical equations?

Chemical compounds

You know that an element is a basic chemical unit. An element is the same as an atom. In certain situations when two or more elements are combined, they will bond together, forming a new substance. The resulting object or combination of elements is called a molecule.

For example, one carbon atom and two oxygen atoms will bond together to form the carbon dioxide molecule. Also, two oxygen atoms will bond together to form an oxygen molecule.

In the case of carbon dioxide, different elements are combining, so the molecule is called a compound. We usually deal with chemical compounds, since there are so many possible combinations.

Thus, a compound is a molecule that is made up of at least two different elements.

Chemical formulas

Chemical formulas (or more correctly: formulae) are designations of molecules and compounds in shorthand notation, similar to that used in Algebra.

Shorthand for elements

Elements can be written as abbreviations or in a shorthand notation. For example, He denotes helium, Fe denotes iron, and Cl denotes chlorine. A chemical formula is writing the elements of a compound, using their abbreviations.

Number of atoms in a molecule

If there is more than one atom of a type in the molecule, the formula shows it by a small number after the symbol. For example, water is H2O, which means there are 2 atoms of hydrogen and one atom of oxygen in the molecule. Carbon dioxide is CO2 , which means there is one atom of carbon and two atoms of oxygen in the molecule.

Number of molecules

To show the number of molecules, a full sized number is located in front of the molecule. For example 4 molecules of carbon dioxide is designated as:

4CO2.

This means there are a total of 4 C atoms and 8 O atoms in the combination. A way to remember this—taken from Algebra—is to think of it as 4 x (CO2).

Complex formulae

Just as in Algebra, you can use parentheses to separate parts in a complex formula. One example is the formula for nitroglycerin, a highly explosive substance.

C3H5(NO3)3

This formula shows that nitroglycerin consists of 3 atoms of C, 5 atoms of H and then 3 molecules of NO3. If the parentheses were not used, you might have a formula like:

C3H5N3O9

The number of atoms for each element would be correct, but it wouldn't help to describe the true structure of the nitroglycerin molecule.

Remember that molecules are 3-dimensional collections of atoms. In more complex molecules--especially in organic substances--the configuration becomes important.

In conclusion

The number of atoms of each element in a chemical formula is designated by the small number behind each element symbol. If there is no number, it is assumed there is only one of that element. A large number in front of a compound designates how many units there are of that compound. Parentheses can be used to designate a special structure, where other molecules are attached to the larger, complex molecule.

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