1. We know that the stability of a half filled orbital (like d5) or a full filled orbital (like d10) is always higher than the other configurations. So, an element or an ion would always prefer the half filled or full filled configuration to gain stability, thus they always try to go somehow to that configuration. So, amongst lanthanoids, Ln (III) compounds are predominant. However, occasionally in solution or in solid compounds, +2 and +4 ions are also obtained due to the extra stability of half filled, empty or fully filled f orbitals.
2. As copper lies below hydrogen in the metal activity series, so it cannot liberate hydrogen gas from dilute acids. Due to which copper has high enthalpy of atomization and low enthalpy of hydration. Hence, E0Cu2+/Cu is positive.
3. We have studied that as we go down a group or towards the bottom of the periodic table, we would expect those elements to be heavier than those at the top. But, due to lanthanide contraction, Zr (At. No. 40) and Hf (At. No. 72) have almost identical radii.
The lanthanide contraction is the ‘greater than expected decrease’ in ionic radii of the elements in the lanthanide series from atomic number 57 - lanthanum, to 71 - lutetium, which results in smaller than otherwise expected ionic radii for the subsequent elements. This is because atomic radii of 4d and 5d transition elements are nearly same and this similarity in size is due to weak shielding of d-electrons. So, we observe that the metallic radii of the third (5d) series of transition metals are same as those of the corresponding members of the second series.
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