Transition metal

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In chemistry, the term transition metal (sometimes also called a transition element) has two possible meanings:
  • The IUPAC definition[1] states that a transition metal is “an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell”.
  • Most scientists describe a “transition metal” as any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table. All elements in the d-block are metals. In actual practice, the f-block is also included in the form of the lanthanide and actinide series.

Jensen[2] has reviewed the history of the terms transition element (or metal) and d-block. The word transition was first used to describe the elements now known as the d-block by the English chemist Charles Bury in 1921, who referred to a transition series of elements during the change of an inner layer of electrons (for example n=3 in the 4th row of the periodic table) from a stable group of 8 to one of 18, or from 18 to 32.[3]

Characteristic properties

There are a number of properties shared by the transition elements that are not found in other elements, which results from the partially filled d shell. These include

  • the formation of compounds whose colour is due to dd electronic transitions
  • the formation of compounds in many oxidation states, due to the relatively low reactivity of unpaired d electrons.[12]
  • the formation of many paramagnetic compounds due to the presence of unpaired d electrons. A few compounds of main group elements are also paramagnetic (e.g. nitric oxide, oxygen)