Transition metal compounds often exhibit vibrant colours due to the presence of d-d electronic transitions. In a transition metal complex, the metal ion has unpaired electrons in its d orbitals. These unpaired electrons can absorb light of specific wavelengths when an electron transitions between different d orbitals. The wavelengths of light absorbed correspond to the colour we observe. The colour of a compound depends on the energy difference between the d orbitals and the nature of the ligands surrounding the metal ion. Different ligands cause different energy differences, leading to different colours.

Examples:

• [Fe(CN)₆]^3-: This complex is deep purple in colour. The d-d transition involves the movement of electrons between the d orbitals, which absorb light in the yellow-green region of the visible spectrum, resulting in the absorption of the complementary colour, purple.
• Cupric chloride (CuCl₂): This compound is typically blue. The blue colour arises from the d-d transition involving the d orbitals of the Cu²⁺ ion.

The titration of a dilute solution of iron(II) ions (Fe²⁺) with a standard solution of potassium permanganate (KMnO₄) is a redox titration. Potassium permanganate acts as the oxidizing agent, and iron(II) ions are oxidized to iron(III) ions. The chemical equation for the reaction is:

5Fe²⁺(aq) + MnO₄^-(aq) + 8H⁺(aq) → 5Fe³⁺(aq) + Mn²⁺(aq) + 4H₂O(l)

To determine the endpoint, a few drops of a suitable indicator are added to the solution. A commonly used indicator is potassium fernicilate (K₃[Fe(CN)₆]). This indicator is initially colourless. As the permanganate is added, it oxidizes the iron(II) ions to iron(III) ions. The iron(III) ions react with the potassium fernicilate to form a deep pink/purple precipitate of iron(III) fernicilate. The endpoint is reached when the solution turns a persistent pink/purple colour, indicating that all the iron(II) ions have been oxidized.

The volume of potassium permanganate solution used to reach the endpoint can then be used to calculate the concentration of the initial iron(II) solution using the stoichiometry of the reaction.

Transition metals generally have high densities due to their metallic bonding. This type of bonding involves delocalisation of electrons within a "sea" of electrons surrounding positively charged metal ions. The close packing of these ions in the crystal lattice, combined with the strong electrostatic attraction between the ions and the delocalised electrons, results in a high density. The relatively small size of the metal ions also contributes to efficient packing.

• Metallic Bonding: Delocalisation of electrons.
• Close Packing: Efficient arrangement of ions in the crystal lattice.
• Strong Electrostatic Attraction: Between positive ions and the electron sea.
• Small Ionic Radii: Facilitates efficient packing.