d-Block Elements — Properties & Key Compounds (KMnO₄, K₂Cr₂O₇)

Why Cr and Cu are exceptions: Extra stability of exactly half-filled ($3d^5$) and completely filled ($3d^{10}$) subshells. One electron shifts from 4s to 3d to achieve these stable configurations.

Ions lose 4s electrons first: When forming ions, 4s electrons are removed before 3d. So ${\text{Fe}}^{2+}$ is $[\text{Ar}]3d^6$ (not $3d^{4}4s^2$), and ${\text{Fe}}^{3+}$ is $[\text{Ar}]3d^5$.

A. Variable Oxidation States

The most characteristic property — arises because 3d and 4s electrons are close in energy and both can participate in bonding.

Trend: Maximum oxidation state = group number (up to Mn, group 7). For later elements (Fe onwards), maximum oxidation state decreases as extra d electrons are harder to remove.

B. Atomic and Ionic Radii

• Atomic radii decrease from Sc to Cr (nuclear charge increases, d electrons shield poorly), then remain nearly constant from Cr to Cu (added electrons go into d, better shielding), then slightly increase at Zn.
• Ionic radii decrease with increasing oxidation state: ${\text{Fe}}^{2+}>{\text{Fe}}^{3+}$ (fewer electrons, more nuclear attraction).

C. Ionisation Enthalpies

• Generally increase across the period (increasing nuclear charge) but with irregularities due to $d^5$ and $d^{10}$ stability.
• Much higher than s-block metals → stronger metallic bonding, higher melting/boiling points.
• Mn has unusually high first IE (stable $d^5$ configuration).

D. Magnetic Properties

Paramagnetism arises from unpaired d electrons. Magnetic moment (in Bohr magnetons, BM):

$$\mu =\sqrt{n(n+2)}\text{ BM}$$

where $n$ = number of unpaired electrons.

E. Colour of Transition Metal Compounds

Coloured compounds arise from d–d transitions: an electron absorbs light (visible) and jumps from a lower to a higher d orbital. The complementary colour is observed.

• ${\text{Cu}}^{2+}$ (aq): blue;   ${\text{Cr}}^{3+}$ (aq): violet/green;   ${\text{Fe}}^{3+}$ (aq): yellow/brown;   ${\text{Mn}}^{2+}$ (aq): very pale pink
• ${\text{Zn}}^{2+}$, ${\text{Sc}}^{3+}$, ${\text{Ti}}^{4+}$: colourless (no d–d transition possible — fully filled or empty d)
• ${\text{MnO}}_4^{-}$ (permanganate): intense purple — due to charge transfer, not d–d transition

F. Catalytic Activity

Transition metals are excellent catalysts because they can adopt variable oxidation states and form intermediate compounds with reactants.

G. Formation of Interstitial Compounds and Alloys

• Interstitial compounds: Small atoms (H, C, N, B) fit into lattice voids → hard, high melting, poor conductors. Example: Steel (Fe + C).
• Alloys: Transition metals mix readily (similar radii, metallic bonding) → stainless steel (Fe, Cr, Ni), brass (Cu, Zn).

Preparation

Step 1 — Fusion: ${\text{MnO}}_2$ fused with KOH in air or ${\text{KNO}}_3$ (oxidising agent) gives green manganate:

$$2{\text{MnO}}_2+4\text{KOH}+{\text{O}}_2\stackrel{\mathrm{\Delta }}{\to }2{\text{K}}_2{\text{MnO}}_4+2{\text{H}}_2\text{O}$$

Step 2 — Electrolytic oxidation (or ${\text{Cl}}_2$ oxidation) of manganate to permanganate:

$$2{\text{K}}_2{\text{MnO}}_4+{\text{Cl}}_2\to 2{\text{KMnO}}_4+2\text{KCl}$$

$$\text{Mn: }+6\to +7\text{ (oxidation)}$$

Properties and Oxidising Action

KMnO₄ is a powerful oxidising agent. Its action depends on the medium:

What KMnO₄ Oxidises (Acidic Medium)

• ${\text{Fe}}^{2+}\to {\text{Fe}}^{3+}$:   ${\text{MnO}}_4^{-}+5{\text{Fe}}^{2+}+8{\text{H}}^{+}\to {\text{Mn}}^{2+}+5{\text{Fe}}^{3+}+4{\text{H}}_2\text{O}$
• Oxalate: ${\text{MnO}}_4^{-}+5{\text{C}}_2{\text{O}}_4^{2-}+16{\text{H}}^{+}\to 2{\text{Mn}}^{2+}+10{\text{CO}}_2+8{\text{H}}_2\text{O}$
• ${\text{SO}}_2$, ${\text{H}}_2\text{S}$, ${\text{Br}}^{-}$, ${\text{I}}^{-}$, ${\text{NO}}_2^{-}$ — all oxidised in acidic medium.

Preparation

Step 1: Roasting chromite ore with ${\text{Na}}_2{\text{CO}}_3$ in air → sodium chromate:

$$4{\text{FeCr}}_2{\text{O}}_4+8{\text{Na}}_2{\text{CO}}_3+7{\text{O}}_2\to 8{\text{Na}}_2{\text{CrO}}_4+2{\text{Fe}}_2{\text{O}}_3+8{\text{CO}}_2$$

Step 2: Acidification converts chromate (yellow, ${\text{CrO}}_4^{2-}$) to dichromate (orange, ${\text{Cr}}_2{\text{O}}_7^{2-}$):

$$2{\text{Na}}_2{\text{CrO}}_4+2{\text{H}}^{+}\to {\text{Na}}_2{\text{Cr}}_2{\text{O}}_7+{\text{H}}_2\text{O}$$

Step 3: Treatment with KCl gives K₂Cr₂O₇ (less soluble, crystallises out).

Chromate ⇌ Dichromate Equilibrium

$$2{\text{CrO}}_4^{2-}+2{\text{H}}^{+}\rightleftharpoons {\text{Cr}}_2{\text{O}}_7^{2-}+{\text{H}}_2\text{O}$$

Adding acid shifts equilibrium right (dichromate, orange). Adding base shifts left (chromate, yellow). This interconversion is a highly tested JEE/NEET concept.

Oxidising Action of K₂Cr₂O₇

Acts as oxidising agent in acidic medium:

$${\text{Cr}}_2{\text{O}}_7^{2-}+14{\text{H}}^{+}+6e^{-}\to 2{\text{Cr}}^{3+}+7{\text{H}}_2\text{O}$$

Colour change: orange (${\text{Cr}}^{6+}$) → green (${\text{Cr}}^{3+}$). Oxidises: ${\text{Fe}}^{2+}\to {\text{Fe}}^{3+}$, ${\text{I}}^{-}\to {\text{I}}_2$, ${\text{H}}_2\text{S}\to \text{S}$, ${\text{SO}}_2\to {\text{SO}}_4^{2-}$.