Chapter 4: Structure of the Atom

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Structure of Atom


If bromine atom is available in the form of, say, two isotopes _{35}^{79}	extrm{Br} (49.7%) and _{35}^{81}	extrm{Br} (50.3%), calculate the average atomic mass of bromine atom.


The average atomic mass is the average mass of the atoms in the naturally occurring sample of the element. The average atomic mass of an element is the sum of the mass of its isotopes each multiplied by its natural abundance. 

The natural abundance of  _{35}^{79}	extrm{Br} and _{35}^{81}	extrm{Br} is 49.7 % and 50.3% respectively. So, the average atomic mass of bromine atom is calculated as shown below:

=79	imes frac{49.7}{100} + 81	imes frac{50.3}{100}

=frac{3926.3}{100} + frac{4074.3}{100}

=39.263 + 40.743=80.006u


Therefore, the average atomic mass of bromine atom is 80.006 u.

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