transition metal

Any of a group of metallic elements that have incomplete inner electron shells and exhibit variable valency – for example, cobalt, copper, iron, and molybdenum. They form a long block in the middle of the periodic table of the elements, between groups 2 and 3. They are excellent conductors of electricity, and generally form highly coloured compounds.

They include most of the hard, dense, and less reactive metals used as building materials, such as iron, lead, and copper. The precious metals, gold, platinum, and silver are also transition metals. They are rare, beautiful and costly, and so unreactive that they can be used as jewellery. Transition metals and their compounds are often used in industry as catalysts. For example, iron is used as a catalyst in the Haber process to produce ammonia. They are also used to make alloys.