Atomic Structure & Mass Spectrometry (OCR A Level Chemistry A): Revision Note

Isotopes & Relative Atomic Mass

• Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.

  • Isotopes can also be described as atoms of the same elements but with different mass numbers

• Therefore, the mass of an element is given as relative atomic mass (A_r) by using the average mass of the isotopes

• The relative atomic mass of an element can be calculated by using the relative abundance values by using the following equation:

  • 

  • The relative abundance of an isotope is either given or can be read off the mass spectrum

Mr from Mass Spectra

• The percentage abundance of the isotopes in an element can be found by the use of a mass spectrometer

• The basic processes of mass spectrometry are:

  • The sample is vapourised

  • The sample is ionised to form positive ions

  • The ions are accelerated

    • Heavy ions move slower / are less deflected

  • The ions are detected as a mass-to-charge ratio, written as 

    • Each ion produces a signal, so the larger the signal, the greater the abundance

• The mass spectra produced can be used to calculate the relative atomic mass of an element and its isotopes

Relative molecular and formula mass

• These are essentially the same ideas 

  • Relative molecular mass is applied to chemicals that have a fixed formula in terms of the number of atoms involved, e.g. ethanol, C₂H₅OH, contains two carbon atoms, six hydrogen atoms and one oxygen

  • Relative formula mass is applied to chemicals that use an empirical formula to represent them, e.g. calcium chloride, CaCl₂, is an ionic lattice containing a ratio of 1 calcium ion : 2 chloride ions throughout the structure

• These terms are often mis-used, with relative molecular mass being applied to any compound regardless of its composition

• The relative molecular and formula mass are both calculated by adding up the relative atomic masses of all the component atoms