(a) What are radioactive isotopes ? Give two examples of radioactive isotopes.

(b) Give any two uses of radioactive isotopes.

(c) An element Z contains two naturally occurring isotopes ${}_{17}{}^{35}\mathrm{Z}$ and ${}_{17}{}^{37}\mathrm{Z}$. If the average atomic mass of this element be 35.5 u, calculate the percentage of two isotopes.

(a) Those isotopes which are unstable due to the presence of extra neutrons in their nuclei and emit various types of radiations are called radioactive isotopes. The radiations emitted are in the form of alpha particles, beta particles and gamma rays.
Common examples of radioactive isotopes are Arsenic−74, Iodine−131 and Cobalt−60.

(b) Uses of isotopes−
(i) Cobalt−60 is used in the treatment of cancer cells. When the high−energy gamma radiations emitted by cobalt−60 isotopes are directed at the cancerous tumours , the cells are burnt.
(ii) Iodine−131 is used as a tracer to determine the rate at which the thyroid gland takes up iodine.

(c) Average atomic mass of Z = 35.5u
Let the % of isotope with mass number 35 be x
Then the % of isotope with mass number 37 will be 100−x.

$$\begin{gathered} \mathrm{Average}\mathrm{atomic}\mathrm{mass}\mathrm{of}\mathrm{chlorine}=35\left[\frac{x}{100}\right]+37\left[\frac{100-x}{100}\right] \\ 35=\frac{35x}{100}+\frac{37\left(100-x\right)}{100} \\ x=75 \end{gathered}$$

It means that the % of the isotope ${}_{17}{}^{35}\mathrm{Cl}$ is 75 and the % of the isotope ${}_{17}{}^{35}\mathrm{Cl}$ is 25.