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i.o.u. an explanation for a.m.u.

Ok, let’s get something straight.

There’s a lot of language to get your head around when you first start out with Chemistry, and it all starts with how the very smallest chemical particle is measured.

In topic 1 of the Chemistry for GAMSAT course, we start from the very basics, so this post is to help you get off to a good start.

It’s the small things….

We are dealing with such small particles, that have very very small masses, so there are some unique terms that have been derived to help us. It will help if you have an understanding of the following terms:

  • atomic mass
  • relative atomic mass (replaces the old term atomic weight)
  • unified atomic mass unit (replaces the old term atomic mass unit)

The following video has a good introduction from first principles, including a quick explanation of the distinction that physicists make between weight and mass. The chemistry definitions start around the three minute mark.


Notice that the video makes no mention of relative atomic mass, or of unified atomic mass unit, which are actually the new terms for the atomic weight and the atomic mass unit respectively.

Although still somewhat shrouded in controversy among chemists, the terms relative atomic mass and unified atomic mass unit (still abbreviated to amu) replace the old terms. However, both terms in each case are still in use and are officially sanctioned by the International Union of Pure and Applied Chemistry (IUPAC)

Out with the old, in with the new:

  • The relative atomic mass is exactly equivalent to the old term atomic weight. Changed in 1961 after controversey arising from the distinction between weight and mass.
  • The unified atomic mass unit is the new term for atomic mass unit, both are abbrevieated to amu.

What does it all mean?

unified atomic mass unit

  • Defined 112 of the mass of an atom of carbon-12.
  • Symbol u.
  • One unified atomic mass unit is approximately the mass of one proton (or one neutron) and is equivalent to 1 g/mol.
  • Has a value of 1.660538921(73)×10−27 kg.

relative atomic mass 

  • Defined as m/mu where m is the average mass of an element and mu is atomic mass constant. (mu = m(12C)/12 which = 1 Da or 1 u).
  • Symbol Ar.
  • This is the number that appears on the periodic table which takes into account natural isotopic abundances.
  • Indicates a ratio-it compares a property of one substance to the same property of another substance. Hence it has no units, it is strictly not a mass but a ratio of two masses.
  • However, for both practical and historical reasons, relative atomic masses (and molecular masses) are almost always quoted in grams per mole (g/mol or g mol−1) in chemistry.

atomic mass

  • Defined as the mass of an atomic particle, sub-atomic particle, or molecule.
  • Symbol ma
  • Commonly expressed in unified atomic mass units (u)
  • Does not take into account natural isotopic abundances.

For another explanation of the atomic mass units, relative atomic mass and natural isotopic abundance, check out the Fuse School videos below:

This is covered in Tutorial 1 of the Chemistry for GAMSAT course. To view a preview of the course click here.

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