Isotopes MCQs Quiz | Class 9

Class IX Science (Code 086), Unit I: Matter – Its Nature and Behaviour. This quiz covers the definition of isotopes, common examples like Hydrogen and Chlorine, and their qualitative applications and importance in fields like medicine and energy. Submit your answers to check your score and download the PDF solution.

Overview of Isotopes

In Class 9 Science (Unit I: Matter), we learn that atoms of the same element can have different masses. These are called isotopes. They are fundamental to understanding atomic structure and have significant practical applications in our daily lives.

1. Definition

Isotopes are atoms of the same element that have:

  • The same atomic number (number of protons).
  • A different mass number (due to a different number of neutrons).

Since the atomic number determines the chemical identity, isotopes belong to the same element and occupy the same position in the periodic table.

2. Common Examples

Isotopes of Hydrogen:

  • Protium (H-1): 1 proton, 0 neutrons. (Most common).
  • Deuterium (H-2): 1 proton, 1 neutron. (Heavy hydrogen).
  • Tritium (H-3): 1 proton, 2 neutrons. (Radioactive).

Isotopes of Chlorine:

Chlorine occurs in nature in two isotopic forms: Cl-35 and Cl-37. They exist in a ratio of approximately 3:1. This is why the average atomic mass of Chlorine is taken as 35.5 u.

3. Properties

  • Chemical Properties: Isotopes have identical chemical properties because they have the same number of valence electrons.
  • Physical Properties: Isotopes have different physical properties (like density, boiling point) because these depend on mass.

4. Applications and Importance

Some isotopes are unstable and radioactive. These radioisotopes have crucial uses:

Isotope Application
Uranium-235 Used as fuel in nuclear reactors.
Cobalt-60 Used in the treatment of cancer.
Iodine-131 Used to treat goitre (thyroid disorders).
Carbon-14 Used in carbon dating to find the age of fossils.

Quick Revision List

  • Isobars vs. Isotopes: Isobars have same mass number but different atomic number (e.g., Calcium and Argon). Isotopes are the reverse.
  • Fractional Mass: Atomic mass is often fractional because it is the weighted average of naturally occurring isotopes.

Extra Practice Questions

  1. Why do isotopes show similar chemical properties? (Ans: Same electronic configuration).
  2. Name the isotope used in detecting blood clots. (Ans: Sodium-24).
  3. Calculate the neutrons in C-14. (Ans: 14 – 6 = 8 neutrons).
  4. Which isotope of Hydrogen is radioactive? (Ans: Tritium).
  5. Are the boiling points of H-1 and H-2 the same? (Ans: No, they differ due to mass).