2.1.1 (c) Relative Isotopic and Atomic Masses

Syllabus

Relative mass

relative isotopic mass (mass compared with 1/12th mass of Carbon-12)

relative atomic mass, A_r, (weighted mean mass compared with 1/12th mass of Carbon-12),

based on the mass of a ¹²C atom, the standard for atomic masses

Weighing atoms or even sub-atomic particles in grams isn't very sensible.

So a relative scale is used.

This is a scale where one thing is compared to another.

The Burj Khalifa in Dubai is 828m tall

The Eiffel Tower is 300m.

If we used the Eiffel Tower as the standard building then its height would be 1.

The Burj would have a height of 2.76

But anything shorter than the Eiffel Tower would have a height less than 1.

We do the same with atomic masses.

The most obvious atom to use as the standard is Hydrogen since it is abundant and the lightest atom.

This seems to make it ideal because all other atoms would have a mass greater than 1.

But Neutrons weigh a little more than Protons.

So the ideal standard atom would be abundant and have an equal number of Protons and Neutrons.

Carbon fits the bill since almost all Carbon atoms are ¹²C - with 6 Protons and 6 Neutrons.

But it can't be given a mass of 1 because that would make the relative mass of ¹H only 0.0833.

So, we use 1/12 of the mass of ¹²C to represent 1.

- is the mass of an atom of an isotope compared to 1/12 of the mass of a ¹²C atom.

Or, since weighing one atom is impossible...

-it is the mass of 1 mole of atoms of an isotope compared to 1/12 of the mass of a mole of ¹²C atoms.

All elements have isotopes.

So, a sample of Neon will contain mostly atoms of ²⁰Ne but also a few ²¹Ne atoms and some ²²Ne atoms.

If we took a simple mean of 20, 21 and 22 we would calculate an average mass of 21 for Neon atoms.

But this is unrealistic because nearly all the atoms are ²⁰Ne - 90.92%.

And there are only 0.26% ²¹Ne and 8.82% ²²Ne.

A more realistic average would take into account these percentages.

This is a weighted mean - and must be between the highest and lowest value

And in this case, it must be closer to a mass of 20.

You may have done the calculation, right, at GCSE.

If not then click here.

Definition: Relative Atomic Mass

The relative atomic mass is the weighted mean mass of 1 mole of an element compared to the mass of 1 mole of ¹²C