(a)(i) Size of halide ions order F⁻<Cl⁻<Br⁻<I⁻. With increasing size, the negative charge is dispersed throughout the ion, and the ion gets stabilized. Thus, the acidity order is HF<HCl<HBr<HI.
(ii) Due to the presence of vacant d-orbitals and the combining forms of sulfur (S8), which are not found in oxygen, the cohesive energy of sulfur is more than oxygen, leading to a large difference in melting and boiling points.
(iii) Due to the absence of d-orbitals, nitrogen cannot extend its octet to form a pentahalide.