Give reasons for each of the following:
(i) Size of trivalent lanthanoid cation decreases with increase in the atomic number.
(ii) Transition metal fluorides are ionic in nature whereas bromides and chlorides are usualy covalent in nature.
(iii) Chemistry of all the lanthanoids is quite similar.

Answer

(i) Cause of lanthanide contraction.
(ii) As electronegativity of halogens decreases in the order `FgtClgtBr`, By , the ionic character of transition metal haldies decrease in the order `M-FgtM-ClgtM-Br`. Hence , fluorides are ionic whereas chloride and bromides are covalent.
(iii) The change in the size of the lanthanoids due to lanthanoid contraction is very small as we produceed from `La(Z=57)` to `Lu(Z=71)`. Hence , their chemical properties are similar . Moreover, their valence shell configuration remains the same because the electrons are added into the inner `4` f-subshell. hence , they show similar chemical properties .