(a). `K_2PtCl_2` is a well known compound whereas corresponding is not known. State a reason for it.
(b). Most of the transition metals do not displace hydrogen from dilute acids. Why?
(c). Why have the transition elements high enthalpy of hydration?
(d). Chromium is a typical hard metal while mercury is a liquid. Explain why.?

Explanation

(a). This is because `Pt^(4+)` is more stable than `Ni^(4+)` as the sum of four ionisation enthalpies of Pt is less than that of Ni.
(b). This is because most of the transition metals have negative oxidation potentials.
(c). This is due to their small size and large nuclear charge. This is so because when we move along any transition series, the nuclear charge increases and size decreases.
(d). Cr has five unpaired d electrons in the d-subshell `(3d^54s^1)`. Hence, metallic bonds are very strong. In Hg, all the d-orbitals are fully filled `(3d^(10)4s^2)`. Hence, the metallic bonding is very weak.