Give a reason for each of the following:
i) The size of trivalent lanthanoid cations decreases with increase in atomic number.
ii) Transition-metal fluorides are ionic in nature, whereas bromides and chlorides are usually covalent.
iii) The chemistry of all the lanthanoids is quite similar.
i) The size of trivalent lanthanoid cations decreases with increase in atomic number.
ii) Transition-metal fluorides are ionic in nature, whereas bromides and chlorides are usually covalent.
iii) The chemistry of all the lanthanoids is quite similar.
Solution
(i) Each added 4f electron shields the added nuclear charge poorly, so the effective nuclear charge felt by the outer shells rises steadily — Ln radii shrink from La to Lu (lanthanoid contraction).
(ii) By Fajans' rules: the small, highly electronegative F is scarcely polarisable, so M–F bonds stay ionic; the larger, more polarisable Cl and Br are distorted by the metal cation, introducing covalent character.
(iii) All lanthanoids show the same dominant +3 state, and the contraction keeps their ionic sizes almost identical — hence very similar chemical behaviour throughout the series.
(ii) By Fajans' rules: the small, highly electronegative F is scarcely polarisable, so M–F bonds stay ionic; the larger, more polarisable Cl and Br are distorted by the metal cation, introducing covalent character.
(iii) All lanthanoids show the same dominant +3 state, and the contraction keeps their ionic sizes almost identical — hence very similar chemical behaviour throughout the series.
CBSE Class 12 Board · 2005 · All India Compartment · Subjective