The Difference Between Atomic Weight and Atomic Mass

Atomic weight and atomic mass are two important concepts in chemistry and physics. Many people use the terms interchangeably, but they don't actually mean the same thing. Take a look at the difference between atomic weight and atomic mass and understand why most people are confused or don't care about the distinction. (If you're taking a chemistry class, it could show up on a test, so pay attention!)

Atomic Mass Versus Atomic Weight

Atomic mass (m_a) is the mass of an atom. A single atom has a set number of protons and neutrons, so the mass is unequivocal (won't change) and is the sum of the number of protons and neutrons in the atom. Electrons contribute so little mass that they aren't counted.

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Atomic Mass Versus Mass Number

By Todd Helmenstine

Atomic weight is a weighted average of the mass of all the atoms of an element, based on the abundance of isotopes. The atomic weight can change because it depends on our understanding of how much of each isotope of an element exists.

Both atomic mass and atomic weight rely on the atomic mass unit (amu), which is 1/12th the mass of an atom of carbon-12 in its ​ground state.

Can Atomic Mass and Atomic Weight Ever Be the Same?

If you find an element that exists as only one isotope, then the atomic mass and the atomic weight will be the same. Atomic mass and atomic weight may equal each other whenever you are working with a single isotope of an element, too. In this case, you use the atomic mass in calculations rather than the atomic weight of the element from the periodic table.

Weight Versus Mass: Atoms and More

Mass is a measure of the quantity of a substance, while weight is a measure of how a mass acts in a gravitational field. On Earth, where we are exposed to a fairly constant acceleration due to gravity, we don't pay much attention to the difference between the terms. After all, our definitions of mass were pretty much made with Earth gravity in mind, so if you say a weight has a mass of 1 kilogram and a 1 weight of 1 kilogram, you're right. Now, if you take that 1 kg mass to the Moon, it's weight will be less.

So, when the term atomic weight was coined back in 1808, isotopes were unknown and Earth gravity was the norm. The difference between atomic weight and atomic mass became known when F.W. Aston, the inventor of the mass spectrometer (1927) used his new device to study neon. At that time, the atomic weight of neon was believed to be 20.2 amu, yet Aston observed two peaks in the mass spectrum of neon, at relative masses 20.0 amu and 22.0 amu. Aston suggested there two actually two types of neon atoms in his sample: 90% of the atoms having a mass of 20 amu and 10% with a mass of 22 amu. This ratio gave a weighted average mass of 20.2 amu. He called the different forms of the neon atoms "isotopes." Frederick Soddy had proposed the term isotopes in 1911 to describe atoms that occupy the same position in the periodic table, yet are different.

Even though "atomic weight" is not a good description, the phrase has stuck around for historical reasons. The correct term today is "relative atomic mass" — the only "weight" part of the atomic weight is that it is based on a weighted average of isotope abundance.