Transition Metals

Transition metals are a class of chemical elements found in the central region of the periodic table. They occupy columns 3 through 12 and include well-known elements such as iron (Fe), copper (Cu), gold (Au), silver (Ag), titanium, chromium and nickel.

What distinguishes this group is the structure of their electrons. The electrons fill an inner region called the 'd-orbital', which remains incomplete. This gives them distinctive chemical properties: multiple oxidation states, coloured compounds, and the ability to function as catalysts.

Definition and boundaries of the group

The standard IUPAC definition refers to an element with an incomplete d-shell, or one capable of forming an ion with an incomplete d-shell. For this reason, zinc (Zn), cadmium and mercury are sometimes excluded from the list, since their d-shells are complete under normal conditions. Group 12 is therefore sometimes considered 'post-transition metal' rather than transition metal, although common usage tends to include it.

Physical properties

Most transition metals are hard, dense and have high melting points. They conduct electricity and heat well. Compared to Group 1 and 2 metals (such as sodium or calcium), they are harder and less reactive with water. The strength of metallic bonding, arising from the d-electrons, accounts for this hardness.

Oxidation states and colour

Each element in this group can display multiple oxidation states. For example, iron forms Fe(II) and Fe(III), while manganese can reach Mn(VII). This is because the energies of the d and s electrons are close, so they can be lost in varying numbers. Many of their compounds are coloured: copper(II) solution is blue, chromium(III) is green, permanganate is deep purple. This colour arises from electronic transitions between the separated d-energy levels when ligands surround the metal centre.

Coordination compounds and catalysis

Transition metals form coordination compounds, in which molecules or ions (ligands) attach to the metal centre. Examples include haemoglobin, which holds iron and binds oxygen, and vitamin B12, which contains cobalt. In industry, this group serves as catalysts: iron in the Haber process for ammonia synthesis, platinum and palladium in vehicle exhaust cleaners, and titanium in polymer manufacture.

Uses and economy

Iron and its alloys (wrought iron, steel) form the foundation of construction and manufacturing. Copper is used in electrical wiring. Gold and silver are used in currency and electronic devices. Chromium and nickel are added to steel to prevent corrosion. Titanium is selected for its low density and high strength, particularly in aircraft and medical equipment.