Relative Atomic Mass and Standard Atomic Weight

What is the Relative Atomic Mass?

The atomic mass of an atom is the mass of that one atom. Different isotopes will have different atomic masses - so how do we know what the average atomic mass of a chemical element is?

We look at the relative atomic mass. This can sometimes be referred to as the atomic weight of a chemical element; however this is old terminology and is not usually used because it can easily become confused with standard atomic weight (which is defined further below). Importantly, the relative atomic mass is sample-specific (i.e. it can change from sample to sample).

How do we calculate the Relative Atomic Mass?

To calculate the relative atomic mass of a sample, we need to know three things:

1. the different naturally occurring isotopes
2. the atomic mass of each of those isotopes (Ma)
3. the abundance of each of those isotopes in a sample (N)

Once you have this information you can calculate the relative atomic mass by:

1. multiplying each isotope's atomic mass by its relative abundance
2. adding these multiplications together; and
3. dividing the result by the number of atoms in the sample.
where N1= number of atoms of isotope 1, Ma1 = atomic mass of isotope 1 etc.

Take carbon for example (as in the video above). In a naturally occurring sample of 100 carbon atoms there are two isotopes: carbon-12 (12 u) and carbon-13 (13.003 u). If ninety-nine atoms in the sample are carbon-12, and one is carbon-13, the sample's relative atomic mass would be calculated as below:

Work through some examples

The video above goes through a few examples of how to calculate the relative atomic mass for a sample. There are more available for you in a video here, along with complementary exercises for you to work through in your own time.

How do Relative Atomic Mass and Standard Atomic Weight differ?

As previously mentioned, the relative atomic mass of a chemical element can vary by sample. This is because there can be variations in the type and abundance of isotopes found in the sample. For example, a sample from the ocean floor may be quite different to a sample from the top of Mount Everest or even from outer space.

While this can be useful, in helping us identify where samples are from etc. it's also useful to have a standard; for example, for samples that we use in the lab. And that's exactly what the standard atomic weight is. It is the mass based on a 'normal' sample. It's the value that you'll see quoted in periodic tables.

In addition to teaching you how to calculate the relative atomic mass for a sample, the video above also helps you practice comparing the calculated atomic mass of a sample with the standard atomic weight to make some basic conclusions.