CHEM C1: Relative Atomic Masses

I won’t be covering stoichiometry & mole calculations that were learnt pre-A-Levels.
For now, I’ll jump straight into new info:

  • Analysing Mass Spectra
  • Calculating Relative Atomic Mass of an Element

Let’s go!

Some definitions that you should already know:

Relative Atomic Massaverage mass of an atom of an element relative to 1/12th of a Carbon-12 atom, where a Carbon-12 atom has a mass of 12 units exactly
Relative Isotopic Massmass of an isotope of an element relative to 1/12th of a Carbon-12 atom, where a Carbon-12 atom has a mass of 12 units exactly
Relative Molecular Massmass of a compound relative to 1/12th of a Carbon-12 atom, where a Carbon-12 atom has a mass of 12 units exactly

We’re already aware that the mass of an atom is measured relative to 1/12th of the mass of one Carbon-12 atom.
But the question is, how do we actually measure the mass of individual atoms?

The answer is through…
Mass Spectrometry
A mass spectrometer is an instrument used to measure the abundance of ions of different masses.

We don’t have to know how a Mass Spectrometer works, but it might be some fun info:
→ [VIDEO]: Simple explanation of the Mass Spectrometer by FranklyChemistry

The MASS SPECTRUM is shown as a graph of relative abundance (number or %) against mass/charge ratio (m/e), like so:

Image result for mass spectrum a levels chemistry chlorine

Given that each ion measured is of the same element, the graph shows different isotopes of that element.

The relative atomic mass of the element itself is found by calculating the MEAN of all the isotopes.

Finding the Atomic Mass of an Element

  1. Multiply the MASS of each isotope by its ABUNDANCE
  2. Add all the calculated values
  3. Divide the sum by the total abundance (which as a percentage is 100%)

Other Resources:

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