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an element A has two isotopes of masses 6&7 in the ratio 1:3 .calculate the atomic mass of the element ?

Most of the elements exists as isotopes which are different atoms of the same element with different mass no. and same atomic no. Therefore the atomic mass of an element must be its average atomic mass.

          If 'a' and 'b' be the atomic masses of the two isotopes of an element ; 'm' and 'n' be the ratio in which these isotopes occur, then average atomic mass of the element is:

Average atomic mass = (a x m + b x n) / ( m+ n)

Here a = 6,  b = 7, m = 1 and  n = 3, then
 
Av. At.mass = {(6 x 1) + (7 x 3)} /  (1 + 3)
                  = 27 / 4
                  = 6.78 u


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