Question

Assertion A : Hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere. Reason R : Hydrogen is the lightest element.

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

Light gases like H₂ and He are rare in the atmospheres of terrestrial planets (like Earth) but abundant in gas giants (like Jupiter) because of the difference in gravitational pull.

Answer 1

The Correct Option is A

The given question includes an assertion (A) and a reason (R) statement. We need to evaluate both statements individually and determine the logical relationship between them to select the correct option.

Assertion (A): Hydrogen is the most abundant element in the Universe, but it is not the most abundant gas in the troposphere.

Reason (R): Hydrogen is the lightest element.

1. Evaluation of Assertion (A):
   • Hydrogen indeed is the most abundant element in the Universe, primarily forming stars through nuclear fusion in their cores.
   • In the Earth's atmosphere, specifically the troposphere, nitrogen (~78%) and oxygen (~21%) are the most abundant gases, making hydrogen not the most abundant there.
   • Therefore, Assertion A is true.
2. Evaluation of Reason (R):
   • Hydrogen is the lightest element, with the atomic number 1 and an atomic mass of approximately 1 amu (atomic mass unit).
   • This lightness contributes to its escape from the Earth's gravitational field into space, explaining its low concentration in the troposphere.
   • Therefore, Reason R is true.
3. Relationship between A and R:
   • While both A and R are true, the reason for hydrogen's scarcity in the troposphere is directly due to its lightness, allowing it to rise and escape Earth's atmosphere.
   • Thus, Reason R offers a correct explanation for why hydrogen is not the most abundant gas in the troposphere, even though it is the most common element in the Universe.

Conclusion: Both Assertion A and Reason R are true, and R is the correct explanation of A. Therefore, the correct option is:

Both A and R are true and R is the correct explanation of A