(a) When a chromite ore (A) is fused with an aqueous solution of sodium carbonate in free excess of air, a yellow solution of compound (B) is obtained. This solution is filtered and acidified with sulphuric acid to form compound (C). Compound (C) on treatment with solution of KCl gives orange crystals of compound (D). Write the chemical formulae of compounds A to D.
(b) Describe the cause of the following variations with reSQPect to lanthanoids and actinoids:
(i) Greater range of oxidation states of actinoids as compared to lanthanoids.
(ii) Greater actinoid contraction as compared to lanthanoid contraction.
(iii) Lower ionisation enthalpy of early actinoids as compared to the early lanthanoids.

Explanation

(a) `A=FeCr_(2)O_(4)" "B=Na_(2)CrO_(4)" "C=Na_(2)Cr_(2)O_(7)" "D=K_(2)Cr_(2)O_(7)`
(b)
(i) 5f, 6d and 7s levels in actinoids are of comparable energies.
(ii) This is due to poorer shieding by 5f electrons in actinoids as compared to shielding by 4f electrons in lanthanoids.
(iii) In actinoids, 5f electrons are more effectively shielded from the nuclear charge than 4f electrons of the correSQPonsing lanthanoids. Since the outer electrons are less firmly held, they are available for bonding in the actinoids.