Isotopes (DP IB Chemistry): Revision Note

Isotopes

What are isotopes?

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

• These are atoms of the same elements but with different mass numbers

  • E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

  • These isotopes could also be written as 12C or C-12, and 14C or C-14 respectively

Isotopes of hydrogen

• Isotopes have the same chemical properties

  • Because they have the same number of electrons and electron configuration

• Isotopes have different physical properties

  • Due to differences in their mass number (from varying numbers of neutrons)

  • Physical properties that can vary include:

    • Mass

    • Density

    • Melting / boiling point

    • Rate of diffusion

Calculating relative atomic mass

What is relative atomic mass?

• The relative atomic mass (A_r) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit

'the average mass of one atom of an element compared to one twelfth of the mass of an atom of carbon-12'

How to calculate relative atomic mass

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

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

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

• The equation to calculate A_r is:

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