Q.Choose correct option.
Thallium shows different oxidation state because _______
a. of inert pair effect b. it is inner transition element c. it is metal d. of its high electronegativity

Every element in groups 13, 14 and 15 can show its group oxidation state (+3, +4, +5 respectively, using all valence electrons) or a state two units lower (+1, +2/-4, +3/-3), and down each group this lower state becomes increasingly stable — the inert-pair effect — because the outer ns2 electron pair is progressively less able to take part in bonding, being poorly shielded by filled inner d (and, from period 6, f) subshells. This is why Tl+1 is more stable than Tl+3, why Pb2+ (PbCl2) is more stable than Pb4+ (PbCl4) while the lighter Ge still favours +4 (GeCl4 over GeCl2), and why heavier group-15 members increasingly favour +3 over +5. A consequence is redox behaviour at the two ends of a group: a species …