Summary

This chapter developed the chemistry of the d-block transition metals and the f-block inner-transition elements as prescribed by the WBCHSE Semester IV syllabus. The first-row (3d) transition metals, Sc through Zn, share the general outer configuration $3d^{1-10}4s^{1-2}$, with chromium ($3d^54s^1$) and copper ($3d^{10}4s^1$) departing from the straightforward filling order to achieve the extra stability of a half-filled or completely filled $3d$ subshell. They occur chiefly as oxide and sulfide ores (chromite for chromium, pyrolusite for manganese, haematite/magnetite for iron) and are characteristically hard, high-melting, alloy-forming metals.

Across the series, atomic and ionic radii fall from Sc to about Cr/Mn, flatten through the middle of the series, and rise again towards Cu and Zn, reflecting the balance between rising nuclear charge and poor $d$-electron shielding. First ionization enthalpy rises only irregularly, with chromium anomalously low and manganese and zinc anomalously high, on account of the differing stability of the resulting $d^n$ configurations. Oxidation states widen from Sc ($+3$ only) to a maximum at Mn ($+2$ to $+7$) before narrowing again to Zn ($+2$ only), because the comparable energies of the $(n-1)d$ and $ns$ orbitals let variable numbers of electrons take part in bonding. Compounds are typically coloured when the metal ion has a partially filled $d$ subshell (permitting a d-d transition) and colourless when $d^0$ or $d^{10}$; transition metals and their oxides are effective catalysts because of their variable oxidation states and large adsorptive surface area; and ions with unpaired $d$ electrons are paramagnetic, with the number of unpaired electrons calculable from the spin-only formula $\mu = \sqrt{n(n+2)}$ BM.

Potassium dichromate is manufactured from chromite ore by oxidative fusion, acidification, and conversion to the potassium salt, and is a strong oxidizing agent in acidic medium ($\text{Cr}_2\text{O}_7^{2-} \rightarrow \text{Cr}^{3+}$, gaining six electrons), governed by a pH-dependent chromate-dichromate equilibrium. Potassium permanganate is manufactured from pyrolusite ore by oxidative fusion to manganate followed by oxidation (chemical or electrolytic) to permanganate, and shows different oxidizing behaviour -- and a different reduction product -- in acidic ($\text{Mn}^{2+}$), neutral ($\text{MnO}_2$) and alkaline ($\text{MnO}_4^{2-}$) media. …