Chapter 5 – The d- and f-Block Elements
1. Introduction
The d- and f-block elements occupy the central and bottom portions of the periodic table.
They are responsible for many important properties such as colour, magnetism, catalytic activity, and variable oxidation states.
2. d-Block Elements (Transition Elements)
Transition elements are those elements which have:
- Partially filled $d$-orbitals in their atoms or ions
- General electronic configuration: $(n-1)d^{1-10}ns^{1-2}$
Example:
Fe (Z = 26): $[Ar]\,3d^6\,4s^2$
3. General Characteristics of Transition Elements
(a) Variable Oxidation States
Transition metals show multiple oxidation states due to similar energies of $(n-1)d$ and $ns$ orbitals.
Example:
Mn shows oxidation states from $+2$ to $+7$
(b) Formation of Coloured Ions
Colour arises due to $d$–$d$ electronic transitions when light is absorbed.
Cu$^{2+}$ → Blue
Cr$^{3+}$ → Green
(c) Magnetic Properties
Presence of unpaired electrons causes paramagnetism.
Magnetic moment:
$$\mu = \sqrt{n(n+2)}\ \text{BM}$$
(d) Catalytic Properties
Transition metals act as catalysts due to:
- Variable oxidation states
- Formation of intermediate complexes
4. Important Compounds of Transition Metals
(a) Potassium Dichromate
$K_2Cr_2O_7$
Uses:
- Oxidising agent
- Laboratory reagent
(b) Potassium Permanganate
$KMnO_4$
In acidic medium:
$$MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$$
5. f-Block Elements
f-Block elements include:
- Lanthanides (4f series)
- Actinides (5f series)
6. Lanthanides
Lanthanides show general configuration:
$[Xe]\,4f^{1-14}5d^{0-1}6s^2$
(a) Oxidation State
Most common oxidation state is $+3$.
(b) Lanthanide Contraction
Gradual decrease in atomic and ionic radii across lanthanides due to poor shielding by $4f$ electrons.
Consequences:
- Similarity between Zr and Hf
- Difficulty in separation
7. Actinides
General configuration:
$[Rn]\,5f^{1-14}6d^{0-1}7s^2$
Actinides show:
- Wide range of oxidation states
- Radioactivity
8. Comparison of Lanthanides and Actinides
| Property | Lanthanides | Actinides |
|---|---|---|
| Oxidation state | Mostly +3 | +3 to +7 |
| Radioactivity | Mostly non-radioactive | All radioactive |
9. Important Exam-Focused Points
- Zn, Cd, Hg are NOT transition elements
- Scandium behaves similar to lanthanides
- Colour requires partially filled d-orbitals
10. Final Revision Checklist
You should be able to:
- Explain trends in oxidation states
- Reason colour and magnetism
- Write important redox reactions
- Explain lanthanide contraction
- Differentiate lanthanides and actinides