Relative Mass (Edexcel A Level Chemistry): Revision Note

Isotopic & Atomic Mass

• The relative mass of an atom uses the carbon-12 isotope as the international standard 

• One atom of carbon-12 has an accepted mass of 1.992646538 x 10^-26 kg

• It is not realistic to work with this value so the mass of a carbon-12 atom is fixed as exactly 12 atomic mass units / 12υ

• The standard mass for atomic mass is 1υ

  • Therefore, the standard mass for comparison is the mass of  of a carbon-12 atom

Relative isotopic mass

• Relative isotopic mass is defined as the mass of an isotope relative to  of a carbon-12 atom

• For A Level Chemistry it is common to work with mass values rounded to one decimal place, for example:

• The accurate relative isotopic mass of nitrogen is 14.00307401 but this is rounded to 14.0

• The accurate relative isotopic mass of oxygen is 15.99491464 but this is rounded to 16.0

Relative atomic mass

• Most elements on the Periodic Table represent a mixture of different isotopes, which is shown as their relative atomic mass (A_r) 

• The relative atomic mass is the weighted mean / average mass of an atom relative to of the mass of a carbon-12 atom

Molecular & Formula Mass

• We have seen previously that the symbol for the relative atomic mass is A_r

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

• The symbol for the relative formula mass is M_r and it refers to the total mass of the substance

  • The term relative formula mass should be used for compounds with giant structures e.g. ionic compounds such as sodium chloride

  • If the substance is molecular you can use the term relative molecular mass

• 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 Calculations Table