Colour: Most transition metal ions are coloured in solution/solid state due to d-d transitions (absorption of visible light excites electron from lower to higher d-orbital). The colour observed is the complementary colour of the light absorbed. (e.g., $Ti^{3+}$ (violet), $V^{3+}$ (green), $Cr^{3+}$ (violet), $Mn^{2+}$ (pale pink), $Fe^{3+}$ (yellow), $Co^{2+}$ (blue/pink), $Ni^{2+}$ (green), $Cu^{2+}$ (blue)).

Catalytic Properties: Many transition metals and their compounds act as catalysts (e.g., V$_2$O$_5$ in contact process, Fe in Haber process, Ni in hydrogenation). Due to variable oxidation states and ability to form unstable intermediates.

Complex Formation: Form a large number of complex compounds due to:

Small size of metal ions.
High ionic charges.
Availability of vacant d-orbitals to accept lone pairs from ligands.

Alloy Formation: Form alloys readily due to similar atomic sizes. (e.g., brass (Cu-Zn), bronze (Cu-Sn), stainless steel (Fe-Cr-Ni)).

Interstitial Compounds: Formed when small atoms (H, C, N) get trapped in the interstitial sites of the metal lattice. They are non-stoichiometric, hard, chemically inert, and retain metallic conductivity.

1.4 Important Compounds of d-Block Elements

Potassium Dichromate ($K_2Cr_2O_7$):
- Preparation: Chromite ore ($FeCr_2O_4$) $\rightarrow$ Sodium chromate $\rightarrow$ Sodium dichromate $\rightarrow$ Potassium dichromate.
- Orange solid, strong oxidising agent in acidic medium: $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$.
- Chromate-dichromate equilibrium: $Cr_2O_7^{2-}$ (orange) $\rightleftharpoons 2CrO_4^{2-}$ (yellow) in acidic/basic medium.
Potassium Permanganate ($KMnO_4$):
- Preparation: Pyrolusite ($MnO_2$) $\rightarrow$ Potassium manganate ($K_2MnO_4$) $\rightarrow$ Potassium permanganate.
- Purple solid, strong oxidising agent.
- In acidic medium: $MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O$ (Equivalent wt. = M/5).
- In neutral/faintly alkaline medium: $MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 + 4OH^-$ (Equivalent wt. = M/3).
- In strongly alkaline medium: $MnO_4^- + e^- \rightarrow MnO_4^{2-}$ (Equivalent wt. = M/1).

2. f-Block Elements (Inner Transition Elements)

2.1 Introduction

Elements in which the differentiating electron enters the $(n-2)$f subshell.
Two series: Lanthanoids (4f series) and Actinoids (5f series).
Placed separately at the bottom of the periodic table.

2.2 Lanthanoids (4f Series)

Elements from Ce (Z=58) to Lu (Z=71).
General electronic configuration: $[Xe] 4f^{1-14} 5d^{0-1} 6s^2$. (Most common is $4f^n 6s^2$, $5d^0$ for many).
Oxidation States: Most common is +3. Some show +2 (Eu, Yb) and +4 (Ce, Pr, Tb) due to stability of empty, half-filled, or fully-filled 4f orbitals.
Lanthanoid Contraction:
- Steady decrease in atomic and ionic radii (M$^{3+}$) from Ce to Lu.
- Reason: Poor shielding effect of 4f electrons, leading to increased effective nuclear charge.
- Consequences:
  - Similarity in properties of 2nd and 3rd row transition elements (e.g., Zr/Hf, Nb/Ta).
  - Difficulty in separation of lanthanoids.
  - Slight increase in basicity of hydroxides from $La(OH)_3$ to $Lu(OH)_3$.
Colour: Most $Ln^{3+}$ ions are coloure