2.1.3 Relative Atomic Mass Calculations

• 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

• Because of this, 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
  • Firstly, find the mass of 100 atoms by multiplying the percentage abundance by the mass of each isotope
  • Secondly, divide by 100 to find the average atomic mass
  • For example, if you have two isotopes A and B:

Answer: 

The correct answer is A

• • Total mass of 100 atoms = (99.76 x 16) + ( 0.04 x 17) + (0.20 x 18) = 1600.44
  • Mass of 1 atom = 1600.44 ÷ 100 = 16.0044 = 16.00 (2 dp)

• Here is another example, but this time using a mass spectrum to obtain the information:

Answer:

• • Total mass of 100 atoms = (19.9 x 10) + (80.1 x 11) = 1080.1
  • Mass of 1 atom = 1080.1 ÷ 100 = 10.801 = 10.80 (2 dp)