• 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 1 ? 12 of a carbon-12 atom

Relative isotopic mass

• Relative isotopic mass is defined as the mass of an isotope relative to? 1 ? 12? 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 (Ar)
• The relative atomic mass is the weighted mean / average mass of an atom relative to? 1 ? 12?of the mass of a carbon-12 atom

• We have seen previously that the symbol for the relative atomic mass is?Ar
• 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?Mr?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?Mr?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