Q.(a)

(i) Interstitial hydrides form by hydrogen occupying gaps in a transition metal lattice (e.g. PdHx), giving non-stoichiometric, metallic hydrides. (ii) Lanthanides fill 4f electrons ([Xe] 4f(1-14) 5d(0-1) 6s2) and actinides fill 5f electrons ([Rn] 5f(1-14) 6d(0-1) 7s2).

Answering part (a), since (a)(i) and (a)(ii) are given as the primary alternative (the OR alternative (b), on the characteristics of internal energy, is not required unless (a) is unanswerable):

(a)(i) Interstitial hydrides: When hydrogen gas is absorbed by certain transition (d-block, and also some f-block) metals, the small hydrogen atoms can fit into the empty interstitial spaces (voids) WITHIN the metal's existing crystal lattice, rather than reacting to form a distinct ionic or covalent compound with a fixed stoichiometry. Because the hydrogen atoms simply occupy pre-existing gaps, these hydrides are usually non-stoichiometric (variable, non-integer composition, e.g. TiH1.5-1.8, PdH0.6-0.8), retain many of the physical properties of the parent metal (metallic lustre, electrical conductivity), and are generally less reactive than ionic (saline) hydrides. A classic example is palladium hydride: palladium metal can absorb up to several hundred times its own volume of hydrogen gas, forming PdHx.

(a)(ii) General electronic configuration: …