CBSE Class 9 Answered

Neon occurs as two isotopes with mass numbers 20 and 22 and its relative atomic mass is 20.2

Weight of Ne-20= 20 amu
Weight of Ne-22= 22 amu

Let the percent abundance of Ne-20 be x and of Ne-22 be 1-x

Now according to the formula, we have:

Relative atomic mass = (exact weight of isotope Ne-20) X (abundance of isotope Ne-20) +  (exact weight of isotope Ne-22) X (abundance of isotope Ne-22)

i.e. (20) (x) + (22) (1- x) = 20.2

Now on solving for x, we get

x = 0.9 (this is for Ne-20)

And similarly, the percent abundance of Ne-22 will be = 1- 0.9 = 0.1