Chemistry S & P Block Elements NUMS 2019
PMDC Verified Question 74 of 104
Which alkaline earth metal makes peroxides?
A
Ba
B
Be
C
Mg
D
Ca
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: Ba
Concept:

The formation of higher oxides (like peroxides \( \text{O}_2^{2-} \) and superoxides) depends on the size and charge density of the metal cation. Large cations with low charge density stabilize large polyatomic anions.

Solution:

  • In Group II-A, as we move down the group, atomic size increases drastically.
  • Beryllium (Be) and Magnesium (Mg) are too small and have high charge densities; they only form normal oxides (e.g., MgO).
  • Calcium (Ca) can form peroxides, but it requires special conditions.
  • Barium (Ba), being near the bottom of the group, has a very large ionic radius. This large size perfectly stabilizes the large peroxide ion (\( \text{O}_2^{2-} \)) in the crystal lattice.
  • Thus, heating Ba in air/oxygen at high temperatures (around 500°C) readily forms Barium peroxide (\( \text{BaO}_2 \)).


Why other options are incorrect:

  • Be and Mg form stable normal oxides. The lattice energy mismatch prevents them from forming stable peroxides.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

Previous Question All S & P Block Elements MCQs Next Question

Want to solve full-length papers under timed exam conditions?

Practice with zero-scroll lockdown sprints, dynamic latency zone timers, live peer selection telemetry, and the automated Amber mistake recovery loop.

Enter Arena Sprint iOS App Android App