Question

a) Atomic sizes increase as we come down a group, but in 4th group of the Periodic Table Zr, Hf have almost the same atomic sizes. Why? 

b) E°(standard electrode potential) values generally become less negative as we move across a transition series, but E° values of Ni²⁺/Ni and Zn²⁺/Zn values are exceptions. Justify.

Answer 1

a) This is due to Lanthanide contraction. It is the phenomenon of regular decrease in atomic size across the lanthanoid series. 

b) Due to the stability of completely filled d¹⁰ conguration of Zn²⁺ its Δ_i HΘ is less. This is responsible for the high negative value of E⁰_{\(Z^{2+}_n/Z_n\)}( – 0.76 V).