Atomic Mass and Atomic Mass Number (Quick Review)

Fingers zeroing in on a depiction of an atom

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Atomic mass and atomic mass number are two important concepts in chemistry. Here's a quick review of what is meant by atomic mass and atomic mass number, as well as how actual particle mass relates to atomic number.

Atomic Definitions

Are Atomic Mass and Atomic Mass Number the Same?

Yes and no. If you are talking about a sample of a single isotope of an element, the atomic mass number and the atomic mass are either very close or else the same. In introductory chemistry, it's probably fine to consider them to mean the same thing. However, there are two cases in which the sum of the protons and neutrons (atomic mass number) is not quite the same as the atomic mass!

In the periodic table, the atomic mass listed for an element reflects the natural abundance of the element. The atomic mass number of the isotope of hydrogen called protium is 1, while the atomic mass number of the isotope called deuterium is 2, yet the atomic mass is listed as 1.008. This is because natural elements are a mixture of isotopes.

The other difference between the sum of protons and neutrons and the atomic mass is due to mass defect. In a mass defect, some of the mass of the protons and neutrons is lost when they bind together to form an atomic nucleus. In a mass defect, the atomic mass is lower than the atomic mass number.