Board-style MCQs and solved questions in the BIEK / Sindh Board pattern. Tap an option to check yourself instantly. Solved questions are at the bottom.
Multiple-choice questions
Solved questions & self-assessment (past papers)
Why does chromium have the configuration [Ar] 3d⁵ 4s¹?
Expected [Ar] 3d⁴ 4s², but one 4s electron shifts into 3d to give a half-filled 3d⁵ set.
A half-filled d-subshell has extra stability (maximum exchange energy + symmetrical charge distribution), so 3d⁵ 4s¹ is lower in energy.
Explain why most transition-metal ions are coloured but Sc³⁺ and Zn²⁺ are not.
Colour comes from a d-d transition: ligands split the d-orbitals; an electron absorbs visible light and jumps the gap, so the complementary colour is seen.
Sc³⁺ = 3d⁰ (empty — no electron to promote); Zn²⁺ = 3d¹⁰ (full — no vacant d-level). Both have no possible d-d transition → colourless.
Calculate the spin-only magnetic moment of Fe³⁺ (3d⁵).
Fe³⁺ = [Ar] 3d⁵ → n = 5 unpaired electrons.
μ = √[n(n+2)] = √[5(7)] = √35 = 5.92 Bohr magnetons — strongly paramagnetic.