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In chemistry, the term transition metal (or transition element) has two possible meanings: *The IUPAC definition defines a transition metal as "an element whose atom has a partially filled ''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.〔Petrucci, R.H.; Harwood, W.S. and Herring, F.G. (2002) ''General Chemistry'', 8th ed, Prentice-Hall, pp. 341-2〕〔Housecroft, C.E. and Sharpe, A.G. (2005) ''Inorganic Chemistry'', 2nd ed, Pearson Prentice-Hall, pp. 20-21〕 In actual practice, the f-block lanthanide and actinide series are also considered transition metals and are called "inner transition metals". English chemist Charles Bury first used the word ''transition'' in 1921 when he 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.〔 These elements are now known as the d-block. ==Classification== In the ''d''-block the atoms of the elements have between 1 and 10 ''d'' electrons. The typical electronic structure of transition metal atoms can be written as ()''ns''2(''n-1'')''d''m, following the Madelung rule where the inner ''d'' orbital is predicted to be filled after the valence-shell ''s'' orbital. This is actually not the case; the 4s electrons are higher in energy than the 3d as shown spectroscopically. An ion such as has no ''4s'' electrons: it has the electronic configuration ()3d6 as compared with the configuration of the atom, ()4s23d6. The elements of groups 3–12 are now generally recognized as transition metals, although the elements La-Lu and Ac-Lr and Group 12 attract different definitions from different authors. # Many chemistry textbooks and printed periodic tables classify La and Ac as Group 3 elements and transition metals, since their atomic ground-state configurations are s2d1 like Sc and Y. The elements Ce-Lu are considered as the "lanthanide" series (or "lanthanoid" according to IUPAC) and Th-Lr as the "actinide" series.〔Petrucci, R. H. ''et al.'' (2002), "General Chemistry", 8th edn, Prentice-Hall, pp. 49–50, 951〕〔Miessler, G. L. and Tarr, D. A. (1999) ''Inorganic Chemistry'', 2nd edn, Prentice-Hall, p. 16〕 The two series together are classified as f-block elements, or (in older sources) as "inner transition elements". # Some inorganic chemistry textbooks include La with the lanthanides and Ac with the actinides.〔Cotton, F.A. and Wilkinson, G. (1988) ''Inorganic Chemistry'', 5th ed., Wiley , pp. 626–7〕〔Housecroft, C. E. and Sharpe, A. G. (2005) ''Inorganic Chemistry'', 2nd ed., Pearson Prentice-Hall, p. 741〕 This classification is based on similarities in chemical behaviour, and defines 15 elements in each of the two series even though they correspond to the filling of an ''f'' subshell which can only contain 14 electrons. # A third classification defines the f-block elements as La-Yb and Ac-No, while placing Lu and Lr in Group 3.〔 This is based on the Aufbau principle (or Madelung rule) for filling electron subshells, in which 4f is filled before 5d (and 5f before 6d), so that the f subshell is actually full at Yb (and No) while Lu (and Lr) has an ()s2f14d1 configuration. However La and Ac are exceptions to the Aufbau principle with electron configuration ()s2d1 (not ()s2f1 as the Aufbau principle predicts) so it is not clear from atomic electron configurations whether La or Lu (Ac or Lr) should be considered as transition metals. Eric Scerri has proposed placing Lu and Lr in group 3 on the grounds of continuous sequences of atomic numbers in an expanded or long-form periodic table.〔Scerri, E.R. (2011) ''A Very Short Introduction to the Periodic Table'', Oxford University Press.〕 Zinc, cadmium, and mercury are sometimes excluded from the transition metals as they have the electronic configuration ()''d''10s2, with no incomplete ''d'' shell.〔Cotton, F. Albert; Wilkinson, G.; Murillo, C. A. (1999). ''Advanced Inorganic Chemistry'' (6th ed.). New York: Wiley, ISBN 0471199575.〕 In the oxidation state +2 the ions have the electronic configuration () d10. However, these elements can exist in other oxidation states, including the +1 oxidation state, as in the diatomic ion . The group 12 elements Zn, Cd and Hg may be classed as post-transition metals in this case, because of the formation of a covalent bond between the two atoms of the dimer. However, it is often convenient to include these elements in a discussion of the transition elements. For example, when discussing the crystal field stabilization energy of first-row transition elements, it is convenient to also include the elements calcium and zinc, as both and have a value of zero against which the value for other transition metal ions may be compared. Another example occurs in the Irving-Williams series of stability constants of complexes. The recent synthesis of mercury(IV) fluoride () has been taken by some to reinforce the view that the group 12 elements should be considered transition metals, but some authors still consider this compound to be exceptional. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「transition metal」の詳細全文を読む スポンサード リンク
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