NCERT Question 10 - Chapter 4 Class 9 - Structure of Atom

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

Answer

We know the percentage of is 49.7%

And the percentage of is 50.3%

We know that,

Average mass of an isotope = (% existence of the isotope 1 x mass of isotope 1) + 

                                                    (% existence of the isotope 2 x mass of isotope 2)

                                                 = (M1 x P1) + (M2 x P2)

                                                 = (79 x 49.7 /100) + (81 x 50.3/100)

                                                 = (79 x 497/1000) + (81 x 503 /1000)

        = (39263/ 1000) + (40743/1000)

                                                 = 39.263 + 40.743

                                                 = 80.006u

      Thus, the average atomic mass of bromine is 80u.