Group 2 atomic size trend

Steps:

• Atomic radius increases down Group 2 as electrons occupy successive shells, shielding outer electrons less effectively from the nucleus.
• The lower metal (R) has greater atomic radius than the upper metal (Q).
• Reactivity and hydroxide solubility also increase down the group, but three statements fit Q as the upper metal if D applies specifically.
• Thus, B distinguishes R as the lower metal.

Why B is correct:

• Atomic radius trend in the periodic table: size increases down a group per the shell-filling principle.

Why the others are wrong:

• A: Higher pH fits the lower metal due to increasing hydroxide solubility, true for Q if Q lower.
• C: Faster reaction with water fits the lower metal due to increasing electropositivity, true for Q if Q lower.
• D: CaCO3 specifically neutralizes soil acidity; true for Q if Q is calcium.

Final answer: B