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