Give explanation for each of the following observations : (a) With the same d-orbital configuration ($d^4$), $\text{Mn}^{3+}$ ion is an oxidising agent whereas $\text{Cr}^{2+}$ ion is a reducing agent. (b) Actinoid contraction is greater from element to element than that among lanthanoids. (c) Transition metals form large number of interstitial compounds with H, B, C and N.