(a) How would you account for the following:
(i) Actinoid contraction is greater than lanthanoid contraction.
(ii) Transition metals form coloured compounds.
(b) Complete the following equation :
`2MnO_(4)^(-)+6H^(+)+5NO_(2)^(-)to`

Explanation

(a) (i) Actinoid contraction is greater than lanthanoid contraction. Contraction in the size of the atom and ions is greater in actinoids compared to lanthanoids because of poor shielding by 5f electrons.
(ii) When an electron from a lower energy d-orbital is excited to a higher energy d-orbital, the energy of excitation corresponds to the frequency of light absorbed. This frequency lies in the visible range. The colour observed corresponds to the complementary colour of the light absorbed.
(b) The completed equation is :
`2MnO_(4)^(-)+6H^(+)+5NO_(2)^(-)to2Mn^(2+)+5NO_(3)^(-)+3H_(2)O`