Assertion (A): The mass of an atom is concentrated in the nucleus. Reason (R): Protons and neutrons are present inside the nucleus. MCQ with Answer and Explanation

Explanation:
HCl is oxidized (Cl⁻ → Cl₂, loss of electrons), so it acts as the reducing agent. MnO₂ is reduced (Mn⁴⁺ → Mn²⁺) and is the oxidizing agent.

Explanation:
The maximum number of electrons in a shell is given by the formula 2n², where n is the principal quantum number. For the N shell, n = 4. Thus, maximum electrons = 2(4)² = 2(16) = 32. The K, L, M, and N shells hold 2, 8, 18, and 32 electrons respectively.

Explanation:
In CaCO₃ decomposition, the oxidation states are: Ca (+2), C (+4), O (-2) in both reactant and products, so no change—not a redox reaction. The others involve changes: Zn(0→+2), Cu(+2→0) in B; Na(0→+1), Cl(0→-1) in C; C(-4→+4), O(0→-2) in D. Redox requires change in oxidation numbers.