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d-Block Elements

Transition Metals: Trends, Properties, and Important Compounds.

By chemca Team • Updated Jan 2026

The d-block elements are those in which the last electron enters the d-orbital. They are often called Transition Elements because they exhibit transitional behavior between s-block (metals) and p-block (non-metals).

1. Electronic Configuration

General Formula

$$ (n-1)d^{1-10} ns^{1-2} $$

Important Anomalies (3d Series):

• Chromium (Z=24): $[Ar] 3d^5 4s^1$ (Half-filled d-orbital stability).
• Copper (Z=29): $[Ar] 3d^{10} 4s^1$ (Fully-filled d-orbital stability).

Definition Check: Zinc ($3d^{10}4s^2$), Cadmium, and Mercury are not considered typical transition elements because they have fully filled d-orbitals in both their ground state and common oxidation states.

2. General Properties

A. Atomic Radii & Lanthanoid Contraction

Radii decrease along the series but become almost constant at the end. However, the radii of 4d and 5d series elements are virtually the same (e.g., Zr $\approx$ Hf).

Reason: Lanthanoid Contraction (Poor shielding by 4f electrons).

B. Oxidation States

They show variable oxidation states due to comparable energies of $(n-1)d$ and $ns$ electrons.

• Most common: +2 (loss of $ns$ electrons).
• Highest OS: Manganese (+7), Osmium (+8).
• Stability depends on $d^0, d^5, d^{10}$ configurations.

C. Standard Electrode Potentials ($E^\circ$)

Trends are irregular. $E^\circ$ values for $M^{2+}/M$ are generally negative (except Cu which is +0.34 V).

Why is $E^\circ_{Cu^{2+}/Cu}$ positive? The high hydration enthalpy of $Cu^{2+}$ does not compensate for the high sum of first and second ionization enthalpies.

3. Special Characteristics

1. Colored Ions: Due to d-d transition of unpaired electrons. (Requires incomplete d-subshell).
Examples: $Ti^{3+}$ (Purple), $Cu^{2+}$ (Blue). $Zn^{2+}, Sc^{3+}$ are colorless ($d^{10}, d^0$).

2. Magnetic Properties: Most are paramagnetic due to unpaired electrons.

$$ \mu = \sqrt{n(n+2)} \text{ B.M.} $$

Where $n$ = number of unpaired electrons.

3. Complex Formation: Due to small size, high charge density, and availability of vacant d-orbitals.

4. Catalytic Properties: Due to variable oxidation states and ability to provide a large surface area for adsorption. (e.g., $V_2O_5$ in Contact Process, Fe in Haber's Process).

4. Important Compounds

A. Potassium Dichromate ($K_2Cr_2O_7$)

Preparation from Chromite ore ($FeCr_2O_4$).

Structure: Chromate ($CrO_4^{2-}$, Tetrahedral, Yellow) $\rightleftharpoons$ Dichromate ($Cr_2O_7^{2-}$, Two tetrahedra sharing corner, Orange).

$$ \underset{\text{Yellow}}{2CrO_4^{2-}} + 2H^+ \rightleftharpoons \underset{\text{Orange}}{Cr_2O_7^{2-}} + H_2O $$

Equilibrium depends on pH (Acidic $\rightarrow$ Dichromate, Basic $\rightarrow$ Chromate).

B. Potassium Permanganate ($KMnO_4$)

Preparation from Pyrolusite ore ($MnO_2$).

Properties: Deep purple color. Strong oxidizing agent in acidic, neutral, and basic media.

Acidic: $$ MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O $$

Neutral/Faintly Alkaline: $$ MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 + 4OH^- $$

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