Magnetism, colour, complexes and catalysis

Several more transition-metal properties come from the d electrons.

Magnetism. A substance with unpaired electrons is paramagnetic: it is pulled into a magnetic field. The strength is given by the spin-only magnetic moment:

µ = √(n(n + 2)) BM

where n is the number of unpaired electrons and BM stands for Bohr magneton. One unpaired electron gives 1.73 BM, two give 2.83, and so on. Count n from the d boxes of the ion (remember: 4s electrons go first).

Colour. Many transition-metal compounds are coloured. A d electron absorbs some visible light and jumps to a slightly higher d orbital: a d-d transition. We see the colours that are not absorbed. Ions with no d electrons (d⁰, such as Sc³⁺, Ti⁴⁺) or a full set (d¹⁰, such as Zn²⁺, Cu⁺) cannot do this, so they are colourless.

Complexes. Transition metals form many complexes, where a metal ion holds molecules such as NH₃ or ions such as CN⁻ around it. Three reasons: the ions are small, they carry a high positive charge, and they have empty d orbitals to accept electron pairs. You will study these fully in the next chapter.

Catalysis. Many transition metals and their compounds are good catalysts (iron in making ammonia, V₂O₅ in making sulphuric acid, nickel in hydrogenation). They can switch between oxidation states and can hold reacting molecules on their surface.

Interstitial compounds. A metal lattice has small gaps between its atoms. Small atoms such as hydrogen, carbon and nitrogen can get trapped in those gaps, as in TiC or Mn₄N. These compounds are hard, have very high melting points and still conduct electricity.

Alloys. Transition metals have similar atomic sizes, so the atoms of one can easily take the place of another in its lattice. That is why they form alloys so readily: brass (copper and zinc), stainless steel, and gold mixed with copper in jewellery.