D and F Block Elements Class 12 Chapter Notes

General Characteristics of d-Block Elements

The d-block elements (Groups 3-12) are often referred to as transition elements. These elements are characterized by the presence of partially filled d-orbitals in their atomic or common ionic states. However, elements like Zinc (Zn), Cadmium (Cd), and Mercury (Hg) do not fit this definition perfectly because they have completely filled d-orbitals in their elementary and most stable oxidation states. Thus, they are technically not considered transition elements, though they are still part of the d-block.

Key characteristics include:

• Electronic Configuration: The general electronic configuration is (n-1)d$^{1-10}$ ns$^{1-2}$. There are several exceptions to this configuration, such as Chromium (Cr) and Copper (Cu), which exhibit d$^{5}$s$^{1}$ and d$^{10}$s$^{1}$ configurations, respectively, due to the extra stability associated with half-filled and completely filled d-orbitals. These exceptions are important for exam purposes.
• Metallic Character: All transition elements are typical metals. They are hard, have high melting and boiling points, high enthalpies of atomisation, and good thermal and electrical conductivity. This is due to the strong metallic bonding facilitated by the large number of unpaired electrons in their d-orbitals, leading to strong interatomic forces.
• Variable Oxidation States: This is a hallmark property. The presence of both (n-1)d and ns electrons, which are close in energy, allows for participation in bonding, leading to a variety of stable oxidation states. For instance, Manganese (Mn) can show oxidation states from +2 to +7. The maximum oxidation state generally increases up to the middle of the series (e.g., Mn) and then decreases.
• Formation of Coloured Ions: Most transition metal compounds are coloured in both solid and aqueous states. This is attributed to d-d transitions, where an electron absorbs energy from visible light and jumps from a lower energy d-orbital to a higher energy d-orbital. The colour observed is complementary to the colour absorbed. For example, hydrated Cu²⁺ ions are blue because they absorb orange-red light.
• Magnetic Properties: Many transition metal ions are paramagnetic due to the presence of unpaired electrons. Paramagnetism increases with the number of unpaired electrons. Diamagnetism occurs when all electrons are paired. Ferromagnetism, a stronger form of paramagnetism, is seen in elements like Fe, Co, Ni.
• Catalytic Properties: Many transition metals and their compounds act as excellent catalysts. This is due to their ability to exhibit variable oxidation states and to form unstable intermediate compounds, providing a new reaction path with lower activation energy. Examples include Fe in Haber process, V₂O₅ in Contact process.
• Formation of Interstitial Compounds: Small atoms like H, C, N can get trapped in the interstitial sites of the transition metal lattices, forming interstitial compounds. These compounds are non-stoichiometric, very hard, have high melting points, and retain metallic conductivity.
• Formation of Complex Compounds: Transition metals readily form coordination compounds (complexes) with various ligands due to their small size, high ionic charge, and availability of vacant d-orbitals to accept electron pairs from ligands.

Key Definitions

Transition Elements

Elements having partially filled d-orbitals in their ground state or in any of their common oxidation states. (Excludes Zn, Cd, Hg)

Lanthanoid Contraction

The steady decrease in atomic and ionic radii of lanthanoids with increasing atomic number due to the poor shielding effect of the 4f electrons.

Actinoid Contraction

Similar to lanthanoid contraction, a gradual decrease in the atomic and ionic radii of actinoids with increasing atomic number, caused by the poor shielding of 5f electrons.

Paramagnetism

The property of substances that are weakly attracted by an external magnetic field due to the presence of unpaired electrons.

Diamagnetism

The property of substances that are weakly repelled by an external magnetic field, possessing only paired electrons.

Disproportionation

A redox reaction in which an element in one oxidation state is simultaneously oxidized and reduced to two different oxidation states.

Interstitial Compounds

Compounds formed when small atoms like H, C, or N occupy the interstitial sites (voids) in the crystal lattice of transition metals.

Worked Examples

• Calculating Magnetic Moment Q: Calculate the spin-only magnetic moment for a d³ ion. A: For a d³ ion, there are 3 unpaired electrons (n=3). Spin-only magnetic moment (μ) = √[n(n+2)] BM μ = √[3(3+2)] = √[3×5] = √15 ≈ 3.87 BM (Bohr Magnetons).
• Identifying Transition Metal Oxidation State Q: What is the oxidation state of Chromium in K₂Cr₂O₇? A: Let the oxidation state of Cr be 'x'. 2(K⁺) + 2(Crˣ) + 7(O²⁻) = 0 2(+1) + 2(x) + 7(-2) = 0 2 + 2x - 14 = 0 2x - 12 = 0 2x = 12 x = +6

Lanthanoids vs. Actinoids

Key Points to Remember

• Exceptions to electronic configuration (Cr, Cu) in d-block are frequent exam questions.
• Zinc, Cadmium, and Mercury are not considered true transition elements due to completely filled d-orbitals (d¹⁰) in their stable oxidation states.
• Variable oxidation states of transition elements arise from the small energy difference between (n-1)d and ns electrons.
• Coloured compounds are due to d-d transitions; the color observed is complementary to the absorbed color.
• Lanthanoid contraction leads to similar radii for elements of the second and third transition series (e.g., Zr and Hf), causing similar chemical properties.
• Transition metals and their compounds are excellent catalysts due to variable oxidation states and surface area.
• Magnetic moment is calculated using the formula μ = √[n(n+2)] BM, where 'n' is the number of unpaired electrons.
• Actinoids show a wider range of oxidation states and greater tendency to form complexes compared to lanthanoids.
• MnO₂ can be used for the preparation of potassium permanganate (KMnO₄), while K₂Cr₂O₇ is prepared from chromite ore.

Exam Strategy & Common Traps

Pay special attention to electronic configurations and their exceptions for the d-block elements. Questions on variable oxidation states, catalytic properties, and the formation of coloured ions are very common. For f-block, thoroughly understand Lanthanoid and Actinoid contractions – their causes and consequences. Be ready to explain why Zn, Cd, Hg are not true transition metals. Practice drawing the structures of common oxyanions like Cr₂O₇²⁻ and MnO₄⁻, and balancing redox reactions involving them. Remember, a clear, concise explanation backed by reasons earns full marks.

Practice Questions with Solutions

• Q: Why do transition elements show variable oxidation states? A: Due to the participation of both (n-1)d and ns electrons in bonding, as their energies are very close.
• Q: Name two transition elements that are exceptions to the general electronic configuration. A: Chromium (Cr) and Copper (Cu).
• Q: What is the main consequence of lanthanoid contraction? A: It makes the elements of the second and third transition series have very similar atomic radii and chemical properties.
• Q: How does the colour of transition metal compounds arise? A: It arises from d-d transitions, where electrons absorb visible light and jump between d-orbitals, with the observed colour being complementary to the absorbed light.

Frequently Asked Questions