How to Calculate Atomic Mass

The mass of an item is listed in the periodic table of the elements.

The mass of a single atom is so small that a unit of measurement has been created to describe it--the atomic mass unit (amu). An atomic mass unit, also called a universal mass unit, is based on the weight of one carbon atom. One amu is equal to 1/12 the mass of a carbon atom, or about the mass of one hydrogen atom. Describing the mass of atoms in atomic mass units is a convenient way to handle a very small number. Chemistry problems call for accurate proportions both in the lab and on paper and the amu allows scientists to calculate these proportions in numbers that are easier to manage. Atomic masses are listed on the periodic table of the elements. These masses are listed with each element and can be used to calculate the mass of a compound. Atomic mass ranges from 1.008 amu for hydrogen to 296 amu for the lab-created element of ununquaternium. In the steps below, we will walk through calculating the atomic mass of a common compound, phosphate.

Things You'll Need

• Periodic table of the elements
• Calculator

Instructions

1. Write out the chemical formula of the compound whose atomic mass you want to calculate. The chemical formula provides the number of atoms in one molecule of a compound (i.e., the chemical formula for water is H2O). For example, the chemical formula of phosphate is H3O4P.

2. Look up the atomic mass for each individual element in the given compound on the periodic table. This number is commonly found just below the letter symbol for the element. For the example, the atomic mass of hydrogen (H) is 1.008 amu per atom, oxygen (O) is 16 amu per atom and phosphorous (P) is 30.97 amu per atom.

3. Multiply the result for every atom in Step 2 by the number of atoms in one molecule of the compound. Phosphate contains three atoms of hydrogen, four atoms of oxygen and one atom of phosphorous in one molecule. So for phosphate the calculations would be as follows; 1.008 amu/atom x 3 hydrogen atoms per molecule = 3.024 amu of hydrogen per molecule, 16 amu/atom x 4 oxygen atoms per molecule = 64 amu of oxygen per molecule and 30.97 amu/atom x 1 phosphorous atom per molecule = 30.97 amu of phosphorous per molecule.

4. Add the numbers calculated for each atom in Step 3 to determine the total amu for one molecule of the compound. So for phosphate, the atomic mass is 3.024 + 64 + 30.97 = 97.99 amu in one molecule of phosphate.

Tips & Warnings

• Verify that the reported significant figures match the number of significant figures provided in the periodic table.

• Always double-check any calculations you perform before reporting your final answer.

• Atomic mass is sometimes referred to as atomic weight. Although mass and weight are not the same thing, the terms “atomic mass” and “atomic weight” do have the same meaning.

Resources

• Read this Article in Spanish
• Los Alamos National Laboratory's Chemistry Division: Periodic Table of the Elements
• Tutorial on Atomic Mass
• ChemBioFinder.com: Database of Chemical Formulas