Atomic Mass Unit and Relative atomic Mass

Atomic Mass Unit and Relative atomic Mass

  • The atomic mass unit is a reference unit to which all elements are compared, and it is expressed as “Relative Atomic Mass” or RAM
  • If RAM for natural Carbon is 12.011, what it means is that the average isotopic mass of carbon atoms are 12.011 times heavier than the reference unit ‘u’ or the atomic mass unit
  • For Li the RAM is 6.941 which means that Li atom is 6.941 times heavier than the ‘u’ Here we are considering the average atomic masses of the different isotopes present in nature
  • So now we have to define the unit ‘u’ or atomic mass unit
  • C-12 is taken as a reference unit, and you divide the C-12 isotope of carbon into twelve equal parts, one part is called ‘Atomic Mass Unit” or ‘u’.
  • All elements are compared to this reference unit to obtain RAM
Calculation of RAM
  • Because relative atomic mass is the average of isotopes, it always will be a fractional value.
  • Natural chlorine is a mixture of two isotopes Cl-35 and Cl-37, 75.8 % is Cl-35 and 24.2 % is Cl-37
  • Now to calculate the RAM of Chlorine we would average the masses of the two isotopes
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  • This mass can be expressed in grams to calculate molar mass or 35.484 grams per mole.
Sample Problems
  1. Calculate the relative atomic mass of naturally occurring magnesium which has the following isotopic composition: Mg–24 = 79.0 %, Mg–25 = 10.0 %, Mg–26 = 11.0 %
  2. Calculate the relative atomic mass of silicon with the following isotopic composition: Si–28 = 92.2 %, Si–29 = 4.7 %, Si–30 = 3.1 %
  3. Calculate the relative atomic mass of silver with the following isotopic composition: Ag–107 = 51.83 %, Ag–109 = 48.17 %

Answers to Sample Problems