(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 ³⁵₁₇Z and³⁷₁₇Z. If the average atomic mass of this element be 35.5 u, calculate the percentage of two isotopes.

Academic Chemistry NCERT Class 9

(a) The isotopes that are unstable due to the presence of extra neutrons in their nuclei and emit various types of radiation are called radioactive isotopes. The radiations emitted are in the form of alpha particles, beta particles, and gamma rays.

Examples of radioactive isotopes: Arsenic−74, Iodine−131 and Cobalt−60.

(b) Uses of isotopes−

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 tumors, the cells are burnt.
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 an isotope with mass number 35 be x

Then the % of an isotope with mass number 37 will be 100−x.

Average atomic mass of chlorine = 35$\frac{x}{100}$+37$\frac{100-x}{100}$

35=$\frac{35x}{100}$+$\frac{37(100-x)}{100}$

x=75

Therefore, the % of the isotope ³⁵₁₇Cl is 75 and the % of the isotope ³⁵₁₇Cl is 25.

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Updated on: 10-Oct-2022

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