How to Calculate Atomic Mass Tutorial

Helium Atom
Helium atom. This helium atom has two protons, two neutrons and two electrons. The atomic mass of this atom would be 4 amu. Credit: Todd Helmenstine

Atomic mass is the mass of a single atom. This is found by adding up all the protons, neutrons and electrons in the atom. One proton has a mass of 1,836 electrons. Since electrons barely change the outcome of the atomic mass, they are generally left out of the calculation. This tutorial will show how to calculate the atomic mass using three different methods, depending on your situation.

1. Look it up.

Sounds easy, but why do work that’s already been done? Instructors will ask for atomic masses when teaching how to use a periodic table. Atomic masses are often listed on the periodic table as a decimal value, usually under the element symbol or name. These numbers need a unit of mass associated with them. Atomic masses are usually given in atomic mass units, or amu, where

1 amu = 1/12 the mass of one Carbon-12 atom.

A common unit of amu is the Dalton. One Dalton = 1 amu. These two units can be used interchangeably. The amu unit is not often used in chemistry. Instead the atomic mass is equal to the number grams of the element in one mole of atoms of the element. The units of measurement for this is grams/mole, or simply g/mol.

Example: Find the atomic mass of nitrogen.

Nitrogen is element number 7. Using the periodic table, we see the atomic mass of nitrogen is 14.01 amu or 14.01 g/mol.

2. Add up the number of neutrons and protons in a single atom.

If the mass of a single atom is needed, simply count the number of protons and neutrons in the atom. This is more properly called the mass number, to distinguish it from standard atomic mass.

Example: What is the atomic mass of a carbon isotope with 8 neutrons?

Carbon is element 6. The element number of an element is the number of protons in the atom’s nucleus. Carbon has 6 protons.
6 protons + 8 neutrons = 14

The atomic mass of this carbon isotope is 14 amu.

3. Calculate a weighted average of all the atoms of an element.

Atomic masses are calculated from weighted averages of all that element’s isotopes based on their abundance. Typically, homework problems will provide you with a sample of an element with a list of all isotopes, their mass and their percent abundance. To solve these problems, follow these two steps:

1. Multiply each isotope’s mass by the percent abundance and divide by each value by 100.
2. Add all these values together.

Example: You are given a sample of carbon that contains 75% carbon-12 and 25% carbon-14. What is the average atomic mass of this sample?
Let’s start with carbon-12. Carbon-12 has an atomic mass of 12. 75% of the sample was carbon-12, so multiply the atomic mass by 75
12 x 75 = 900.
Divide this answer by 100
900/100 = 9

Carbon-14 has an atomic mass of 14. 25% of the sample was this isotope. Multiply the atomic mass of the isotope by 25.
14 x 25 = 350
Divide this by 100
350/100 = 3.5

Add these two values together
9 + 3.5 = 12.5

The average atomic mass of this sample was 12.5 amu.

This should help you with any atomic mass homework problems you may encounter.