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

Step 1: Hydrogen makes up about 75% of the baryonic mass of the universe. However, Earth's troposphere is dominated by $N_2$ (78%) and $O_2$ (21%). Statement A is true.
Step 2: Hydrogen is indeed the lightest element (atomic mass $\approx 1$). Statement R is true.
Step 3: Because it is so light, Hydrogen molecules have high thermal velocities that exceed Earth's escape velocity. Consequently, most primordial hydrogen escaped into space, which is why it's rare in our atmosphere. Thus, R explains A.