Question

Boron occurs in nature in the form of of two isotopes having mass number $10and11$. Their relative abundance is $20\%and80\%$respectively. What is the average atomic mass of boron ?

Answer 1

1. Two isotopes of boron ($\mathrm{B}$) = ${}_5{}^{10}\mathrm{B}\mathrm{and}{}_5{}^{11}\mathrm{B}$
2. Relative abundance of ${}_5{}^{10}\mathrm{B}$ = $20\%$ and mass number = 10
3. Relative abundance of ${}_5{}^{11}\mathrm{B}$ = $80\%$ and mass number = 11
4. Therefore, average atomic mass = $(0.2\times 10)+(0.8\times 11)=10.8u$
5. Hence average atomic mass of boron = $\mathit{10}\mathit{.}\mathit{8}\mathit{}\mathit{u}$