1. Introduction & Position in the Periodic Table

The d-block of the periodic table contains elements of Groups 3 to 12 in which the inner penultimate $(n-1)d$ subshell orbitals are progressively filled across four long periods ($3d, 4d, 5d, 6d$). The f-block consists of elements in which $4f$ and $5f$ orbitals are progressively filled, placed in a separate two-row panel at the bottom of the periodic table.

The Four Transition Series

• 3d Series (First Transition Series): Scandium ($Sc, Z=21$) to Zinc ($Zn, Z=30$). (Filling of $3d$ orbitals).
• 4d Series (Second Transition Series): Yttrium ($Y, Z=39$) to Cadmium ($Cd, Z=48$). (Filling of $4d$ orbitals).
• 5d Series (Third Transition Series): Lanthanum ($La, Z=57$), Hafnium ($Hf, Z=72$) to Mercury ($Hg, Z=80$). (Filling of $5d$ orbitals).
• 6d Series (Fourth Transition Series): Actinium ($Ac, Z=89$), Rutherfordium ($Rf, Z=104$) to Copernicium ($Cn, Z=112$). (Incomplete/Filling of $6d$ orbitals).

2. Electronic Configurations of d-Block Elements

The general outer electronic configuration of d-block elements is $(n-1)d^{1-10} ns^{1-2}$, where $(n-1)$ stands for the inner (penultimate) d-orbitals and $n$ is the outermost principal shell.

3. General Physical Properties & Periodic Trends (d-Block)

3.1 Metallic Characteristics & Lattice Structures

Nearly all transition metals exhibit typical metallic properties: high tensile strength, ductility, malleability, high thermal and electrical conductivity, and metallic lustre. With the exceptions of $Zn, Cd, Hg,$ and $Mn$, they adopt typical metallic crystal lattices at normal temperatures:

3.2 Melting Points & Enthalpies of Atomisation (NCERT Fig 4.1 & 4.2)

3.4 Variation in Atomic and Ionic Radii (NCERT Fig 4.3)

3.5 Density Trends

From Titanium ($Z=22, d=4.1 \text{ g cm}^{-3}$) to Copper ($Z=29, d=8.9 \text{ g cm}^{-3}$), there is a steady increase in density. This is due to the decrease in atomic radius coupled with a simultaneous increase in atomic mass across the period. Density drops slightly at Zinc ($7.1 \text{ g cm}^{-3}$) due to its larger atomic radius.

3.6 Ionisation Enthalpies ($\Delta_i H^\circ$)

• General Trend: First ionisation enthalpy generally increases across each series due to increasing nuclear charge, but the increase is not as steep as in p-block elements because inner $d$-electrons shield outer $s$-electrons.
• Irregularities in $\Delta_i H_2^\circ$ and $\Delta_i H_3^\circ$:
  • Second IE ($\Delta_i H_2^\circ$): Unusually high for $Cr$ ($3d^5 \rightarrow 3d^4$) and $Cu$ ($3d^{10} \rightarrow 3d^9$) because removing the second electron disrupts stable half-filled or fully-filled $d$-orbitals.
  • Third IE ($\Delta_i H_3^\circ$): Exceptionally high for $Mn^{2+}$ ($3d^5 \rightarrow 3d^4$) and $Zn^{2+}$ ($3d^{10} \rightarrow 3d^9$). Conversely, $\Delta_i H_3^\circ$ for $Fe^{2+}$ ($3d^6 \rightarrow 3d^5$) is comparatively low because it yields the stable half-filled $3d^5$ configuration ($Fe^{3+}$).

4. Oxidation States & Standard Electrode Potentials ($E^\circ$)

4.1 Variety of Oxidation States

One of the most notable features of transition metals is their display of variable oxidation states. This variability arises because the energy gap between $(n-1)d$ and $ns$ orbitals is extremely small, allowing electrons from both subshells to participate in bond formation.

4.2 Standard Electrode Potentials ($E^\circ$) (NCERT Fig 4.4)

The standard electrode potential ($E^\circ$) for the $M^{2+}/M$ couple measures the tendency of solid metal to form hydrated divalent cations in aqueous solution. It is NOT determined by ionization energy alone, but by a thermochemical balance of three energy terms:

4.3 Trends in Stability of Halides and Oxides

• Halides: Highest oxidation states are attained in tetrahalides ($TiX_4$), pentafluorides ($VF_5$), and hexafluorides ($CrF_6$). Fluorine stabilizes high oxidation states due to high lattice energy ($CoF_3$) or high bond enthalpy terms ($VF_5, CrF_6$). Fluorides are unstable in low oxidation states (e.g., $VX_2$). Beyond $Mn$, no metal forms trihalides except $FeX_3$ and $CoF_3$.
• Copper Iodide Anomaly: $Cu^{2+}$ oxidises iodide ion ($I^-$) to iodine ($I_2$): $$\mathbf{2Cu^{2+} + 4I^- \longrightarrow Cu_2I_2(s) + I_2}$$ Therefore, $CuI_2$ does not exist.
• Disproportionation of $Cu^+$ in Water: Many $Cu(I)$ compounds disproportionate in aqueous media: $$\mathbf{2Cu^+(aq) \longrightarrow Cu^{2+}(aq) + Cu(s)}$$ Reason: Hydration enthalpy of $Cu^{2+}(aq)$ is much more negative than $Cu^+(aq)$, which more than compensates for the second ionization energy of copper.
• Oxides: Oxygen stabilizes highest oxidation states even better than fluorine due to its ability to form multiple bonds ($\text{M=O}$). Highest oxide is $Mn_2O_7$ (covalent green oil). Beyond Group 7, no higher oxides than $Fe_2O_3$ are known.
  • Acidic/Basic Trend: As oxidation state increases, ionic character decreases and covalent/acidic character increases.
  • $V_2O_3$ (Basic) $\rightarrow V_2O_4$ (Less basic, gives $VO^{2+}$) $\rightarrow V_2O_5$ (Amphoteric, gives $VO_2^+$ in acid and $VO_4^{3-}$ in alkali).
  • $CrO$ (Basic) $\rightarrow Cr_2O_3$ (Amphoteric) $\rightarrow CrO_3$ (Acidic).

5. Important Characteristics and Phenomena

5.1 Magnetic Properties

Transition metal ions generally exhibit magnetic behavior due to unpaired electrons. When a magnetic field is applied, two main behaviors are observed: Diamagnetism (repelled by magnetic field, all paired electrons) and Paramagnetism (attracted by magnetic field, unpaired electrons). Extreme paramagnetism is called Ferromagnetism ($Fe, Co, Ni$).

5.2 Formation of Coloured Ions

Most transition metal compounds display vibrant colors in solid or aquated states.

5.3 Formation of Complex Compounds

Transition metals form a vast array of coordination complexes (such as $[Fe(CN)_6]^{3-}, [Fe(CN)_6]^{4-}, [Cu(NH_3)_4]^{2+}, [PtCl_4]^{2-}$). This tendency is due to:
1. Comparatively small ionic size and high nuclear charge density.
2. Availability of vacant d-orbitals of suitable energy to accept lone pairs donated by ligands.

5.4 Catalytic Properties

Transition metals and their compounds are widely used as industrial catalysts. Reasons include:
1. Ability to adopt variable oxidation states and form unstable intermediate compounds, lowering activation energy.
2. Providing a large surface area with free valencies for reactant molecules to adsorb onto.

5.5 Formation of Interstitial Compounds

Interstitial compounds are formed when small non-metal atoms like Hydrogen ($H$), Carbon ($C$), or Nitrogen ($N$) get trapped inside the vacant interstitial spaces of metal crystal lattices. They are usually non-stoichiometric (e.g., $TiC, Mn_4N, Fe_3H, VH_{0.56}, TiH_{1.7}$).

• High Melting Points: Higher than pure metals.
• Extreme Hardness: Some borides approach diamond in hardness.
• Conductivity: Retain metallic electrical conductivity.
• Inertness: Chemically inert.

5.6 Alloy Formation

An alloy is a blend of metals (solid solution). Transition metals readily form alloys with one another (e.g., Ferrous alloys, stainless steel, Manganese steel) and with non-transition metals (Brass: $Cu+Zn$, Bronze: $Cu+Sn$).
Reason: Their atomic radii are within 15% of each other, allowing atoms of one metal to substitute easily into the crystal lattice of another without major distortion.

6. Important Compounds of Transition Metals ($K_2Cr_2O_7$ & $KMnO_4$)

6.1 Potassium Dichromate ($K_2Cr_2O_7$)

Potassium dichromate is an orange crystalline solid, widely used as an oxidizing agent and primary standard in volumetric analysis.

6.2 Potassium Permanganate ($KMnO_4$)

Potassium permanganate is a dark purple (almost black) crystalline solid. It is an exceptionally powerful oxidizing agent in acidic, neutral, and alkaline media.

7. The f-Block Elements: Lanthanoids

The f-block consists of two inner transition series: Lanthanoids (filling of $4f$ subshell, 14 elements from Cerium $Ce, Z=58$ to Lutetium $Lu, Z=71$) and Actinoids (filling of $5f$ subshell, 14 elements from Thorium $Th, Z=90$ to Lawrencium $Lr, Z=103$).

7.1 Lanthanoid Contraction (NCERT Fig 4.6)

7.3 Chemical Reactivity & Mischmetall

8. The f-Block Elements: Actinoids

The Actinoid series consists of 14 elements from Thorium ($Th, Z=90$) to Lawrencium ($Lr, Z=103$), involving the progressive filling of $5f$ orbitals.

• Electronic Configuration: General formula $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$.
• Radioactivity: ALL actinoids are strictly radioactive. Earlier members have long half-lives, but later members have half-lives ranging from a day to 3 minutes ($Lr$).
• $5f$ Orbital Exposure: Unlike $4f$ orbitals in lanthanoids, $5f$ orbitals in actinoids are less buried and extend more into space, allowing $5f$ electrons to participate in bonding to a much greater extent.
• Greater Range of Oxidation States: Actinoids exhibit a much wider range of oxidation states (up to $+7$ in $Np$ and $Pu$) because the $5f, 6d,$ and $7s$ energy levels are of comparable energies.
• Actinoid Contraction: The steady size contraction across actinoids is GREATER from element to element than lanthanoid contraction because $5f$ electrons exert even POORER shielding than $4f$ electrons.

9. Applications of d- and f-Block Elements

• Structural Materials: Iron and steels are foundational construction materials ($Cr, Mn, Ni$ added for corrosion resistance and strength).
• Pigments & Batteries: $TiO_2$ in pigment industry; $MnO_2$ in dry cell batteries; $Zn$ and $Ni/Cd$ in rechargeable batteries.
• Coinage Metals: Group 11 metals ($Cu, Ag, Au$). UK coins are copper-coated steel or $Cu/Ni$ alloy.
• Industrial Catalysts: $V_2O_5$ (sulfuric acid), $Fe$ (ammonia), $Ni$ (hydrogenation), Ziegler-Natta $TiCl_4+Al(CH_3)_3$ (polythene), $PdCl_2$ (Wacker process), $AgBr$ (photography).

10. High-Yield NCERT Solved Examples & Practice Problems