Relative atomic mass

• What is relative atomic mass?


  The average mass of all the atoms of an element, compared to 1/12th the mass of carbon-12 atom.
• Why is the relative atomic mass of some elements on the periodic table not a whole number?


  Elements can have different forms called isotopes
• Why does chlorine have a mass number of 35.5?


  It has 2 different isotopes, chlorine-35 and chlorine-37
• Relative atomic mass formula


  (% of isotopes 1 x mass of isotope 1) + (% of isotope 2 x mass of isotope 2) + …/100
• How to calculate relative atomic mass


  1. Multiply the percentage abundance of each isotope by its mass
  2. Add these together
  3. Divide the total abundance, if asked to use percentage multiply this number by 100