Atomic Mass Versus Mass Number

There is a difference between the meanings of the chemistry terms atomic mass and mass number. One is the average weight of an element and the other is the total number of nucleons in the atom's nucleus.

• Atomic mass is also known as atomic weight. Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes.
• The mass number is a count of the total number of protons and neutrons in an atom's nucleus.

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The Difference Between Atomic Weight and Atomic Mass

By Anne Marie Helmenstine, Ph.D.

Atomic Mass and Mass Number Example

Hydrogen has three natural isotopes: ¹H, ²H, and ³H. Each isotope has a different mass number.

¹H has 1 proton; its mass number is 1. ₂H has 1 proton and 1 neutron; its mass number is 2. ₃H has 1 proton and 2 neutrons; its mass number is 3. 99.98% of all hydrogen is ¹H. It is combined with ₂H and ₃H to form the total value of atomic mass of hydrogen, which is 1.00784 g/mol.

Atomic Number and Mass Number

Be careful you don't confuse atomic number and mass number. While the mass number is the sum of the protons and neutrons in an atom, the atomic number is only the number of protons. The atomic number is the value found associated with an element on the periodic table because it is the key to the element's identity. The only time the atomic number and mass number are the same is when you are dealing with the protium isotope of hydrogen, which consists of a single proton. When considering elements in general, remember the atomic number never changes, but because there may be multiple isotopes, the mass number may change.