Chapter 4: The d- and f-Block Elements
Key Topics: Electronic Configurations, General Properties of Transition Elements, Lanthanoids & Actinoids, Magnetic Properties, Potassium Dichromate, and Potassium Permanganate.
1. General Electronic Configurations
d-Block Elements (Transition Elements):
$$(n-1)d^{1-10} \, ns^{1-2}$$
f-Block Elements (Inner Transition Elements):
$$(n-2)f^{1-14} \, (n-1)d^{0-1} \, ns^2$$
2. Magnetic Properties & Formulae
Spin-Only Magnetic Moment Formula ($\mu$):
$$\mu = \sqrt{n(n+2)} \quad \text{B.M.}$$
Where:
$$n = \text{Number of unpaired electrons}$$
$$\text{B.M.} = \text{Bohr Magneton}$$
3. Lanthanoids vs Actinoids
| Property | Lanthanoids | Actinoids |
|---|---|---|
| Filling Subshell | Differentiating electron enters $4f$ orbital. | Differentiating electron enters $5f$ orbital. |
| Oxidation States | Principally $+3$, occasionally $+2$ and $+4$. | Shows wider range of oxidation states ($+3, +4, +5, +6, +7$). |
| Radioactivity | Non-radioactive (except Promethium, $\text{Pm}$). | All elements are radioactive. |
| Complex Formation | Less tendency to form complexes. | Greater tendency to form complexes. |
| Contraction Effect | Lanthanoid Contraction (due to shielding of $4f$). | Actinoid Contraction (greater contraction due to poorer shielding of $5f$). |
4. Important Chemical Equations & Oxidation Reactions
Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$) Oxidation in Acidic Medium:
$$\text{Cr}_2\text{O}_7^{2-} + 14 \text{H}^+ + 6 e^- \rightarrow 2 \text{Cr}^{3+} + 7 \text{H}_2\text{O}$$
Oxidation of Iodide to Iodine by Dichromate:
$$6 \text{I}^- \rightarrow 3 \text{I}_2 + 6 e^-$$
Overall Ionic Equation:
$$\text{Cr}_2\text{O}_7^{2-} + 14 \text{H}^+ + 6 \text{I}^- \rightarrow 2 \text{Cr}^{3+} + 7 \text{H}_2\text{O} + 3 \text{I}_2$$
Potassium Permanganate ($\text{KMnO}_4$) Reduction in Acidic Medium:
$$\text{MnO}_4^- + 8 \text{H}^+ + 5 e^- \rightarrow \text{Mn}^{2+} + 4 \text{H}_2\text{O}$$
Oxidation of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ by Permanganate:
$$5 \text{Fe}^{2+} \rightarrow 5 \text{Fe}^{3+} + 5 e^-$$
Overall Ionic Equation:
$$\text{MnO}_4^- + 8 \text{H}^+ + 5 \text{Fe}^{2+} \rightarrow \text{Mn}^{2+} + 5 \text{Fe}^{3+} + 4 \text{H}_2\text{O}$$
Potassium Permanganate Reduction in Neutral/Faintly Alkaline Medium:
$$\text{MnO}_4^- + 2 \text{H}_2\text{O} + 3 e^- \rightarrow \text{MnO}_2 + 4 \text{OH}^-$$