Atomic Mass Versus Mass Number

Atomic Mass and Mass Number Don't Mean the Same Thing

Atomic mass vs. mass number

ThoughtCo / Kaley McKean

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.

Key Takeaways: Atomic Mass Versus Mass Number

  • The mass number is the sum of the number of protons and neutrons in an atom. It is a whole number.
  • The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.
  • Atomic mass value sometimes change over time in publications as scientists revise the natural isotope abundance of elements.

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.

View Article Sources
  1. Klein, David R. Organic Chemistry. 3rd ed., John Wiley & Sons, Inc., 2017.