Relative Atomic Mass (Oxford AQA IGCSE Chemistry)

Relative Atomic Mass

• Relative atomic mass has the symbol Ar

• This is the average mass of the atoms of an element measured relative to carbon-12 

• For example:

  • Carbon has an A_r of 12 and hydrogen has an A_r of 1 on the periodic table

  • So, 1 atom of carbon has the same mass as 12 atoms of hydrogen

Comparing hydrogen and carbon

• The relative atomic mass of any element is shown on the periodic table

  • It is the mass number

• The mass number that is shown on the periodic table is an average value of all the isotopes of an element

  • This is calculated from the mass number and relative abundances of all the isotopes of a particular element

• The isotopes of carbon are carbon-12, carbon-13 and carbon-14

  • But, the percentage of carbon-12 is so high that the average value of all isotopes shown on the periodic table is 12

Using isotopes for the A_r of carbon

• The isotopes of chlorine are chlorine-35 and chlorine-37

  • The percentage of chlorine-35 is roughly three quarters of all chlorine atoms

  • So, the average value of all chlorine atoms shown on the periodic table is 35.5

Using isotopes for the A_r of chlorine