Introduction to d- and f-Block Elements
The d-block elements (groups 3-12) and f-block elements (lanthanoids and actinoids) are collectively known as transition elements. These elements play crucial roles in various industrial, biological, and technological applications.
Why Study Transition Elements?
- They exhibit variable oxidation states
- They form colored compounds
- They show paramagnetic behavior
- They act as catalysts in many reactions
- They form complex compounds
d-Block Elements (Transition Elements)
Position in Periodic Table
The d-block occupies the large middle section flanked between s- and p-blocks. Four series of transition elements exist:
- 3d series: Sc (21) to Zn (30)
- 4d series: Y (39) to Cd (48)
- 5d series: La (57), Hf (72) to Hg (80)
- 6d series: Ac (89), Rf (104) to Cn (112)
Definition: Transition elements are defined as those which have incomplete d subshell either in their ground state or in any of their common oxidation states.
Electronic Configurations
General electronic configuration: (n-1)d1-10 ns1-2
| Element | Atomic Number | Electronic Configuration | Special Feature |
|---|---|---|---|
| Cr | 24 | 3d5 4s1 | Half-filled stability |
| Cu | 29 | 3d10 4s1 | Fully-filled stability |
| Zn | 30 | 3d10 4s2 | Not a transition element |
NEET/JEE Tip: Remember the exceptions in electronic configurations - Cr and Cu have half-filled and fully-filled d-orbitals respectively for extra stability.
Question: Silver atom has completely filled d orbitals (4d10) in its ground state. How can you say that it is a transition element?
Answer: Silver exhibits +2 oxidation state where it has incompletely filled d-orbitals (4d9), hence it is considered a transition element.
General Properties of d-Block Elements
Physical Properties
- High melting and boiling points
- High tensile strength
- Malleable and ductile
- Good conductors of heat and electricity
- Metallic lustre
Note: Zn, Cd, and Hg have relatively low melting points and are volatile compared to other transition metals.
Atomic and Ionic Radii
In a given series, atomic and ionic radii decrease from left to right due to poor shielding by d-electrons.
Important: The atomic radii of 4d and 5d series elements are very similar due to lanthanoid contraction.
Ionization Enthalpies
Ionization enthalpies increase gradually across the series due to increasing nuclear charge. However, the increase is not as steep as in main group elements.
Key Points:
- First ionization energy increases only slightly across the series
- Second and third ionization energies show irregular trends
- Breaks occur at configurations like d5 and d10 due to extra stability
Question: Why do the transition elements exhibit higher enthalpies of atomization?
Answer: Due to large number of unpaired electrons in their atoms, they have stronger interatomic interaction and hence stronger bonding between atoms.
Chemical Properties of d-Block Elements
Oxidation States
Transition elements exhibit variable oxidation states due to similar energies of (n-1)d and ns orbitals.
| Element | Common Oxidation States |
|---|---|
| Sc | +3 |
| Ti | +2, +3, +4 |
| V | +2, +3, +4, +5 |
| Cr | +2, +3, +6 |
| Mn | +2, +3, +4, +6, +7 |
| Fe | +2, +3 |
| Co | +2, +3 |
| Ni | +2, +3 |
| Cu | +1, +2 |
| Zn | +2 |
NEET/JEE Tip: Manganese exhibits the largest number of oxidation states (+2 to +7) in the first transition series.
Trends in Standard Electrode Potentials
The E° values for M2+/M couples become less negative across the series, with Cu having a positive value.
Important: The positive E° value for Cu explains its inability to liberate H2 from acids.
Question: Why is Cr2+ reducing and Mn3+ oxidizing when both have d4 configuration?
Answer: Cr2+ is reducing as its configuration changes from d4 to d3 (half-filled t2g level). Mn3+ is oxidizing as it changes from d4 to d5 (half-filled configuration with extra stability).
Magnetic Properties
Transition elements and their compounds show paramagnetic behavior due to the presence of unpaired electrons.
Magnetic moment, μ = √[n(n+2)] BM
where n = number of unpaired electrons
| Ion | Configuration | Unpaired Electrons | Magnetic Moment (BM) |
|---|---|---|---|
| Sc3+ | 3d0 | 0 | 0 |
| Ti3+ | 3d1 | 1 | 1.73 |
| V2+ | 3d3 | 3 | 3.87 |
| Cr2+ | 3d4 | 4 | 4.90 |
| Mn2+ | 3d5 | 5 | 5.92 |
| Fe2+ | 3d6 | 4 | 4.90 |
| Co2+ | 3d7 | 3 | 3.87 |
| Ni2+ | 3d8 | 2 | 2.84 |
| Cu2+ | 3d9 | 1 | 1.73 |
| Zn2+ | 3d10 | 0 | 0 |
NEET/JEE Tip: Remember the spin-only formula for magnetic moment calculation. It's a frequently asked concept.
Formation of Colored Ions
Transition metal ions are often colored due to d-d transitions. When an electron jumps from a lower energy d-orbital to a higher energy d-orbital, it absorbs light in the visible region.
| Ion | Configuration | Color |
|---|---|---|
| Ti3+ | 3d1 | Purple |
| V3+ | 3d2 | Green |
| Cr3+ | 3d3 | Violet |
| Mn2+ | 3d5 | Pink |
| Fe2+ | 3d6 | Green |
| Fe3+ | 3d5 | Yellow |
| Co2+ | 3d7 | Pink |
| Ni2+ | 3d8 | Green |
| Cu2+ | 3d9 | Blue |
| Zn2+ | 3d10 | Colorless |
Note: Ions with d0 and d10 configurations are colorless as no d-d transitions are possible.
Catalytic Properties
Transition metals and their compounds act as catalysts due to:
- Ability to adopt multiple oxidation states
- Ability to form complexes
- Providing large surface area for adsorption
Important Catalysts:
- V2O5: Contact Process for H2SO4
- Fe: Haber's Process for NH3
- Ni: Hydrogenation of oils
- Pt: Oxidation of NH3 to NO
Formation of Complex Compounds
Transition metals form complex compounds due to:
- Small size and high nuclear charge
- Availability of vacant d-orbitals
- High charge density
Examples: [Fe(CN)6]3-, [Cu(NH3)4]2+, [CoF6]3-
Interstitial Compounds and Alloys
Interstitial Compounds
Formed when small atoms (H, C, N) are trapped in the crystal lattice of transition metals.
Properties:
- High melting points
- Very hard
- Retain metallic conductivity
- Chemically inert
Alloy Formation
Transition metals form alloys readily due to similar atomic sizes.
Important Alloys:
- Stainless steel: Fe, Cr, Ni
- Brass: Cu, Zn
- Bronze: Cu, Sn
- German silver: Cu, Zn, Ni
Important Compounds of Transition Elements
Potassium Dichromate (K2Cr2O7)
Preparation:
- Fusion of chromite ore with Na2CO3 in presence of air:
4FeCr2O4 + 8Na2CO3 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2
- Acidification of Na2CrO4:
2Na2CrO4 + 2H+ → Na2Cr2O7 + 2Na+ + H2O
- Conversion to K2Cr2O7:
Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
Properties and Uses:
- Strong oxidizing agent in acidic medium
- Used in leather industry
- Used as primary standard in volumetric analysis
Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O (E° = 1.33 V)
Potassium Permanganate (KMnO4)
Preparation:
- Fusion of MnO2 with KOH and KNO3:
2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O
- Disproportionation of manganate in acidic/neutral solution:
3MnO42- + 4H+ → 2MnO4- + MnO2 + 2H2O
Oxidizing Reactions:
- In acidic medium: MnO4- reduced to Mn2+
- In neutral/alkaline medium: MnO4- reduced to MnO2
NEET/JEE Tip: Remember the color changes in KMnO4 titrations - pink to colorless in acidic medium, pink to green in alkaline medium.
f-Block Elements (Inner Transition Elements)
Lanthanoids
14 elements following lanthanum (Ce to Lu). General electronic configuration: [Xe] 4f0-14 5d0-1 6s2
Characteristics:
- Silvery white soft metals
- +3 is the most common oxidation state
- Show lanthanoid contraction
- Form colored ions
- Paramagnetic in nature
Actinoids
14 elements following actinium (Th to Lr). General electronic configuration: [Rn] 5f0-14 6d0-2 7s2
Characteristics:
- Radioactive elements
- Show a wide range of oxidation states
- Show actinoid contraction
- Chemistry is more complex than lanthanoids
Lanthanoid Contraction: The gradual decrease in atomic and ionic radii from La to Lu due to poor shielding by 4f electrons. This causes similar sizes of elements in 4d and 5d series.
Comparison Between Lanthanoids and Actinoids
| Property | Lanthanoids | Actinoids |
|---|---|---|
| Common oxidation state | +3 | +3, +4, +5, +6 |
| Radioactivity | Few are radioactive | All are radioactive |
| Contraction | Lanthanoid contraction | Actinoid contraction |
| Binding of 4f/5f electrons | 4f electrons are bound more strongly | 5f electrons are less strongly bound |
| Complex formation | Less tendency | Greater tendency |
Applications of d- and f-Block Elements
- Iron and steel: Most important construction materials
- TiO2: Used in pigment industry
- MnO2: Used in dry battery cells
- Catalysts: V2O5 in contact process, Fe in Haber process
- Coinage metals: Cu, Ag, Au
- Lanthanoids: Used in alloy steels, television screens, catalysts
Important Questions for NEET/JEE
1. Why do transition elements show variable oxidation states?
Due to similar energies of (n-1)d and ns orbitals, electrons from both can participate in bond formation.
2. Why are Zn, Cd, and Hg not considered transition elements?
They have completely filled d-orbitals in both ground state and common oxidation states.
3. What is lanthanoid contraction? What are its consequences?
Gradual decrease in atomic and ionic radii from La to Lu due to poor shielding by 4f electrons. Consequences: Similar sizes of 4d and 5d series elements, difficulty in separation of Zr and Hf.
4. Why is Cu+ unstable in aqueous solution?
Cu+ undergoes disproportionation: 2Cu+ → Cu2+ + Cu. The high hydration enthalpy of Cu2+ makes this reaction favorable.
5. Why is the highest oxidation state of a metal exhibited in its oxide or fluoride only?
Due to small size and high electronegativity, oxygen or fluorine can oxidize the metal to its highest oxidation state.