How to Count Atoms in Molecules

Each molecule of water (H2O) contains three atoms.

The chemical formula of a substance gives the type and number of atoms within the chemical. The type of atom is called the element and is written in the chemical formula using its atomic symbol. Each element differs from other elements by the number of protons in its nucleus. The number of atoms for each element is written as a subscript after the symbol for each element. No subscript is written when there is only one atom of the elements. Thus, hydrogen gas (H2) contains two atoms of hydrogen. Water (H2O) contains two atoms of hydrogen and one atom of oxygen.

Things You'll Need

• Chemical formula
• Periodic Table of the Elements
• Pencil
• Paper

Instructions

1. Identify the different elements in the compound. Elemental symbols are letter abbreviations for the elements and may be referenced from the Periodic Table of the eElements. For example, hydrogen peroxide, H2O2, contains two elements--hydrogen and oxygen. Ammonium sulfate, (NH4)2SO4, contains four elements--nitrogen, hydrogen, sulfur, and oxygen.

2. Write the subscript after each atomic symbol above the symbol. The subscript gives the number of atoms for each element in the chemical formula. For elements without a subscript, the number of atoms is one. Hydrogen peroxide from the previous example contains two atoms of hydrogen and two atoms of oxygen.

3. Multiply the sum of the atoms within any parenthesis by the subscript after the parenthesis. If no subscript is present after the parenthesis, then multiply by one. The two after the parenthesis around the ammonium ion in ammonium sulfate indicates two ions, each containing five atoms--one nitrogen and four hydrogen. This produces ten atoms (5 x 2) for the ammonium ion in ammonium sulfate.

4. Add the number of atoms outside the parenthesis to the number of atoms within the parenthesis to determine the total number of atoms in the chemical. Ammonium sulfate then has a total of 15 atoms--ten atoms from the two ammonium ions plus one sulfur and four oxygen atoms (10 + 1 + 4 = 15).

References

• NDT: Chemical Formula
• Frostburg State University: Molecules and Compounds
• Elmhurst College: Compounds

Resources

• Periodic Table of the Elements