Atomic Structure & Mass Spectrometry (OCR AS 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)

  • A different number of neutrons

• Isotopes are also described as atoms of the same element but with different mass numbers

• The mass of an element is given as relative atomic mass (A_r)

  • This is the weighted average mass of its isotopes

• The relative atomic mass of an element can be calculated from its relative isotopic abundances, using the following steps:

  1. Multiply each isotope’s mass by its abundance

  2. Add the results together

  3. Divide by the total abundance (usually 100)

• The equation for this is:

• Relative abundance values are either provided or can be read from a mass spectrum

Mr from mass spectra

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

• In A-Level Chemistry, mass spectra are interpreted assuming all ions are singly charged (+1)

• The basic processes of mass spectrometry are:

  1. The sample is vapourised

  2. The sample is ionised to form positive ions

  3. The ions are accelerated

     • Heavy ions move more slowly and are less deflected

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

     • Each ion produces a signal proportional to its abundance

     • So, a larger signal means a greater abundance

• A mass spectrum can be used to calculate the relative atomic mass of an element

Relative molecular and formula mass

• These terms are closely related and both involve calculating mass from atomic contributions: 

  • Relative molecular mass refers to substances with simple molecules

    • For example, ethanol (C₂H₅OH) contains:

      • 2 carbon atoms

      • 6 hydrogen atoms

      • 1 oxygen atom

  • Relative formula mass refers to substances with giant structures

    • For example, calcium chloride (CaCl₂) is an ionic lattice with a repeating ratio of:1 calcium ion to 2 chloride ions

• These terms are often misused

  • Relative molecular mass is commonly and incorrectly applied to all compounds

• Both the relative molecular mass and relative formula mass are calculated by adding up the relative atomic masses (Aᵣ) of all the atoms present