Atomic Mass, Mass Number and Unified Atomic Mass

Back to Chemistry Grade 9/10

This video is a basic chemistry video that covers the concepts below. It aims to meet the GCSE (high school grade 9/10) higher-tier requirements in Chemistry.

Check out the bottom of this page for associated resources.

What is the actual mass of an atom?

The atomic mass of an atom is its actual mass - this is in contrast to mass number, which is the number of subatomic particles in the atom's nucleus (what gives the atom its mass).

Because atoms are so small, if we use units like 'grams' to report their mass, we get really unruly numbers that are hard to grapple with (see the video for an example). Instead, we use a system called the unified atomic mass unit (or u for short).

Using the unified atomic mass unit, we are able to compare the atomic masses of different atoms without having to use long small numbers - and that's usually what we want to do: to be able to work out if one atom is heavier than another, not necessarily know their actual mass.

What does 1u equate to?

1u is equal to 1.660538921×10−27 kg. Carbon-12 is the carbon isotope with a mass number of 12 (i.e. it has 6 neutrons).

The atomic mass of carbon-12 is 12 u.

It is not a coincidence that the numbers are exactly the same. One unit of the unified atomic mass system is defined as 1/12th of the mass of carbon-12.

That is, we use carbon-12 as the standard by which we compare all other atoms in the universe.

How do Atomic Mass and Mass Number compare for other atoms?

Although the atomic mass of an atom is not the same as its mass number, it's usually a very similar number. For example, the atomic mass of oxygen is 15.9994 ± 0.0004 u, and its mass number is 16.

The video above compares the unified atomic mass of a few elements and isotopes.

Why are Atomic Mass and Mass Number so similar?

Because the mass number is the sum of the number of protons and neutrons in an atom, and the mass of protons and neutrons are very close to 1u.

Check out these related pages for other atomic mass related trivia/knowledge: