Ar & Mr (Cambridge (CIE) O Level Chemistry): Revision Note

Relative Masses

Relative Atomic Mass

• The symbol for the relative atomic mass is A_r

• The relative atomic mass for each element can be found in the Periodic Table along with the atomic number

• The relative atomic mass is shown underneath the atomic symbol and is larger than the atomic number (except for hydrogen where they are the same)

• Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms 

• The carbon-12 is used as the standard atom and has a fixed mass of 12 units 

• The masses of all other atoms are compared against this atom

• Relative atomic mass (A_r) can therefore be defined as:

  • The average mass of the isotopes of an element compared to 1/12^th of the mass of an atom of ¹²C 

• The relative atomic mass of carbon is 12

  • The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon 

  • The relative atomic mass of hydrogen is 1 which means it has one twelfth the mass of one carbon-12 atom 

Relative molecular (formula) mass

• The symbol for the relative molecular mass is M_r and it refers to the total mass of the molecule

  • The symbol M_r is used for both

    • Relative molecular mass and relative formula mass are the same quantity, and only the name changes with the type of substance

  • Both A_r and M_r are ratios, so they never carry a unit

• To calculate the M_r of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

• Relative formula mass is used when referring to the total mass of an ionic compound

Relative Formula Mass Calculations Table