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1.17 Relative atomic mass

 1.17 Activity 2. Calculating relative atomic mass.

 Students should:

1.17 be able to calculate the relative atomic mass of an element (Ar) from isotopic abundances.

 Sorting by mass

A mass spectrometer is an instrument which can separate and sort the particles in a sample of an element according to their mass. The output from a mass spectrometer is known as a mass spectrum.  For an element the mass spectrum tells us the percentage of each isotope which can be found in a naturally occurring sample. This data then allows us to calculate the relative atomic mass for the element under investigation.  

The relative atomic mass of an element is an average value of the mass of the individual isotopes in a naturally occurring sample of the element


The mass spectrum of boron

 Mass and abundance

The image shown here is a mass spectrum of the element boron. It shows the relative number of isotopes in a naturally occurring sample of the element. 

We  can calculate Ar values using this isotopic abundance data:

For Boron two isotopes exist, Boron 10 and Boron 11. The mass spectrum shows that 20% of boron atoms are Boron -10 and 80% are Boron -11.

The weighted mean is calculated as follows:

The relative atomic mass of boron is a weighted mean of all the isotopes in a naturally occurring sample of the element.

 1.17 Activity 3. You do the Maths

The mass spectrum of Zinc
The mass spectrum of Magnesium
The mass spectrum of Germanium
  • Look at the mass spectra for Zinc, Magnesium and Germanium and try to estimate the relative atomic mass for each element.
  • Use the data on the mass spectra to calculate the relative atomic masses for those three elements

 Answers: Ar of Zinc, Ar of Germanium, Ar of Magnesium

 Activity 4.

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