Concept:Density is the ratio of atomic mass to atomic volume. In a group, mass generally increases faster than volume, causing density to increase downward.
Solution:- Note on Historical Accuracy: The provided answer key for this past paper states Option A is correct, implying a strictly linear increase in density: Li < Na < K < Rb.
- While Lithium is indeed the least dense (0.53 g/cm³), the true chemical reality is that Potassium (0.86 g/cm³) is anomalously less dense than Sodium (0.97 g/cm³) because of a massive volume expansion due to the presence of empty 3d orbitals in K.
- However, based on the historical source material's explanatory notes which explicitly dictate "Li < Na < K < Rb as mass increases", we must select Option A to match the examiner's intended (albeit flawed) logic.
Why other options are incorrect:- Options B, C, and D are either mathematically reversed (listing heaviest first) or skip elements randomly.
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.