Why transition metals form coloured, magnetic compounds
The d-block elements, or transition metals, fill their (n−1)d orbitals and lie between groups 2 and 13. They show variable oxidation states because the energies of their ns and (n−1)d electrons are close; manganese ranges from +2 to +7. Many of their ions are coloured because d–d electron transitions absorb visible light, and those with unpaired electrons are paramagnetic, with magnetic moment μ = √(n(n+2)) BM. They form complexes and interstitial compounds and act as catalysts.
Important compounds include potassium dichromate, K₂Cr₂O₇, and potassium permanganate, KMnO₄, both strong oxidising agents used in titrations. The f-block contains lanthanoids, which fill 4f orbitals, and actinoids, which fill 5f orbitals. Across the lanthanoids, atomic and ionic radii decrease steadily, a trend called lanthanoid contraction, caused by poor shielding by 4f electrons. Their usual oxidation state is +3. Actinoids are radioactive and show a wider range of oxidation states.
Why it matters
Iron and its alloys build our infrastructure, copper carries electricity, and titanium is used in aircraft and medical implants. Transition metal catalysts, such as iron in the Haber process and nickel in hydrogenating oils, are vital in industry. Lanthanoids are used in magnets, phone screens and LED phosphors, and uranium fuels nuclear reactors.
Key terms
- Transition element
- An element with partly filled d orbitals in its atom or common ions.
- Variable oxidation state
- The ability of an element to show more than one oxidation number in its compounds.
- Lanthanoid contraction
- The steady decrease in atomic and ionic size across the lanthanoid series.
- Paramagnetism
- Attraction to a magnetic field caused by unpaired electrons.