What is Atomic Mass?

atomic mass

Core Concepts

In this article you will learn about the different things that contribute to atomic mass. What makes up an atom, and how to determine atomic masses.

Topics covered in other articles

Introduction to Atomic Mass Number

Atoms of any element contain a certain amount of protons. The protons within the nucleus of an atom are what make an element an element. Ex. all atoms with one proton are hydrogen atoms. In chemistry the amount of protons in an atom is referred to as its atomic number. This is usually denoted by the letter “Z.” An atom is usually made up of protons neutrons and electrons, but since electrons are generally so small only protons and neutrons account for the mass of an element. The number of protons, Z, and the number neutrons make up the mass number, denoted as the letter A. This is what is used to calculate an atoms mass number.

mass number (A) = protons + neutrons

If you know how many neutrons an unknown element has (which you normally would from the periodic table) than you can always calculate the number of protons, and since protons never differ in an element you can figure out the identity of the unknown element.

Iron on the periodic table

Look at this example of the element Iron (Fe). Iron has 26 protons and if we use the formula

mass number = protons + neutrons

We find that Fe has about 30 neutrons in its nucleus.


For us to completely understand atomic mass we have to first understand isotopes. Since we learned before that the number of protons in the nucleus of an atom CANNOT differ if it is to remain as the same element, but the number of neutrons can scientists were able to discover something known as isotopes. Frederick Soddy and Ernest Rutherford found that some elements exhibited similar chemical properties but slightly varying atomic masses, this lead them to the discovery of isotopes.

Isotopes are simply atoms of the same element that have the same number of protons (meaning that they have the same chemical properties) but they had different numbers of neutrons. Neutrons are subatomic particles with weights similar to that of a proton but they have no electrical charge. Since isotopes of an element have the same number of protons, they share the same atomic number, Z, and are in the same position of the periodic table.

It is important to remember that the number of electrons are what determine the chemical behavior of an element. The arrangement of electrons in isotopes of the same element remain the same.

Percent Abundance

Percent abundance refers to the proportion of an isotope in a sample of that specific element. It’s important to remember that most elements exist as a mixture of multiple isotopes that have different masses. Percent abundance is useful in expressing the actual abundance of each isotope in the natural occurrence of that element.

What is Atomic Mass?

Atomic mass, sometimes referred to as atomic weight, is just an average of the different masses of an element. This number usually has units of amu or u. The atomic mass of one isotope of an element is the sum of that isotopes subatomic particles: protons, neutrons, and electrons (usually of negligible mass.)

Each of the atomic masses, the number underneath an element, is simply a weighted average of the masses of each isotope of that element. Since percent abundance of each of these isotopes vary in nature there is a particular formula for calculating the overall atomic mass of an element:

Atomic mass = (Isotope1 x Abundance1) + (Isotope2 x Abundance2) +……

In this formula “Isotope” is the mass of that specific isotope and abundance is the fractional or percent abundance of that isotope in nature.


Carbon-12 is a specific isotope of carbon, it is very important in determining the atomic masses of other elements. The amu is defined as one twelfth the mass of a carbon-12 atom.

The carbon 12 atom is so important because it is exactly 12 amu. This provides scientists with a standard for measuring the masses of other atoms and molecules. The choice to use carbon-12 as this standard was made because it is chemically stable, and has a integer value for its atomic mass.