Isotopes (HL IB Chemistry)

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
• The way to represent an isotope is to write the chemical symbol (or the word) followed by a dash and then the mass number
  • 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

Using the chemical symbols of hydrogen to determine the number of subatomic particles in each isotope

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 definition of relative atomic mass is:

'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
• For example, if you have two isotopes A and B:
  • Find the mass of 100 atoms by multiplying the percentage abundance by the mass of each isotope:

total mass of 100 atoms = (% abundance_A x mass_A) + (% abundance_B x mass_B) 

• • Then divide by 100, to find the average / relative atomic mass:

relative atomic mass =