Atomic Masses

Relative atomic mass is the mass of an atom of an element compared to the mass of an atom of carbon 12.

The carbon-12 isotope was chosen as an international standard for the measurement of relative mass.

Relative isotopic mass is the weighted mass of an atom of an element compared to one twelfth ofthe mass of an atom carbon-12

Relative atomic mass is the samenumber as the mass number;

• for exmple Oxygen - 16 the relative atomic number is 16!
• and for sodium - 23 the relative atomic number is 23!

If a sample of bromine has 53.00% of bromine - 79 and 47.00% of bromine - 81
Detemine the relative atomic mass of bromine.

Answer:

53.00 divided by 100 = 0.53 X 79.00 = 41.87

47.00 divided by 100 = 0.47 X 81.00 = 38.07

38.07 + 41. 87 = 79.94

You can find the relative molecular mass Mr by adding together the relative atomic masses, for example

• Cl2  = 35.5 X 2 = 71.0
• H20 = (1.0 X2) + 16.0 = 18.0