Relative Atomic Mass (AQA GCSE Chemistry): Revision Note

What is relative atomic mass?

• Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (A_r) is used

• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element

• The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom

• All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units

• For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

How to calculate relative atomic mass (Ar)

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

• The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present

$A_{\mathrm{r}}=\frac{(\%\mathrm{of}\mathrm{isotope}\mathrm{A}\mathrm{x}\mathrm{mass}\mathrm{of}\mathrm{isotope}\mathrm{A})+(\%\mathrm{of}\mathrm{isotope}\mathrm{B}\mathrm{x}\mathrm{mass}\mathrm{of}\mathrm{isotope}\mathrm{B})}{100}$

• So, if there were 3 isotopes present then the equation would read:

$\frac{(\%\mathrm{of}\mathrm{isotope}\mathrm{A}\mathrm{x}\mathrm{mass}\mathrm{of}\mathrm{isotope}\mathrm{A})+(\%\mathrm{of}\mathrm{isotope}\mathrm{B}\mathrm{x}\mathrm{mass}\mathrm{of}\mathrm{isotope}\mathrm{B})+(\%\mathrm{of}\mathrm{isotope}\mathrm{C}\mathrm{x}\mathrm{mass}\mathrm{of}\mathrm{isotope}\mathrm{C})}{100}$