(a) Account for the following
(i) `Ti^(3+)` is coloured whereas `Sc^(3+)` is colourless in aqueous solution.
(ii) `Cr^(2+)` is a strong reducing agent.
(b) Write two similarities between chemistry of lanthanoids and actinoids.
(c ) Complete the following ionic equation:
`3MnO_4^(2-) +4H^+ to`

Explanation

(i) `Sc^(3+)` has no electron in the d-orbitals. There is no possibility of excitation to the electron from low energy d- orbital to high energy d-orbital. There is no absorption of radiation and hence no colour.
`Ti^(3+)` has one electron in 3d orbital. It adsorbs radiation in excitation from lower to higher d-orbital. It is therefore coloured.
(ii) `Cr^(2+)` is a strong reducing agent. That means it easily oxidised to `Cr^(3+)` in which it has a configuration `3d^3`. It is a stable electronic configuration because it is half filled in the orbitals of `t_(2g)` which are three in number.
(b) Similarities between the chemistry of lanthanoids and actinoids:
(i) Variations in the magnetic susceptibilities of actinoids with the number of unpaired f electrons is roughly parallel to teh corresponding results for lanthanoids.
(ii) The lanthanoid and actinoid contractions have extended effects on sizes and properties of elements succeeding them.
(c ) `3MnO_4^(2-)+4H^(+) to 2MnO_4^(-)+MnO_2+2H_2O`