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A '''transition element''' is a [[chemical element]] element whose [[atomic electron configuration]] describing  the ground (lowest energy) state has an incompletely filled ''d'' sub-shell. Here ''d'' stands for an [[Atomic orbital#Solutions of the atomic Schrödinger equation|atomic orbital]] with [[angular momentum (quantum)|angular momentum]] quantum number ''ℓ'' = 2.  That is, the electron configuration of transition element atoms contains  (''nd'')<sup>''k''</sup>,  1 &le; ''k'' &le; 9,  where ''n'' is a [[principal  quantum number]], ''n'' = 3, 4, 5. The incomplete electronic ''d'' subshell gives rise to some characteristic magnetic properties ([[paramagnetism]] and [[ferromagnetism]]) and crystals and solutions of transition metal complexes that are brightly colored.
A '''transition element''' is a [[chemical element]] element whose [[atomic electron configuration]] describing  the ground (lowest energy) state has an incompletely filled ''d'' sub-shell. Here ''d'' stands for an [[Atomic orbital#Solutions of the atomic Schrödinger equation|atomic orbital]] with [[angular momentum (quantum)|angular momentum]] quantum number ''ℓ'' = 2.  That is, the electron configuration of transition element atoms contains  (''nd'')<sup>''k''</sup>,  1 &le; ''k'' &le; 9,  where ''n'' is a [[principal  quantum number]], ''n'' = 3, 4, 5, see table II. The incomplete electronic ''d'' subshell gives rise to some characteristic magnetic properties ([[paramagnetism]] and [[ferromagnetism]]) and crystals and solutions of transition metal complexes that are brightly colored.


The first three series of the transition elements are shown in two tables below.   
The first three series of the transition elements are shown in table I.   
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Rows and columns of the [[Periodic Table of Elements]] containing transition elements </h4>
I. Rows and columns of the [[Periodic Table of Elements]] containing transition elements </h4>
<center> The [[atomic number]] ''Z'' is between brackets</center>
<center> The [[atomic number]] ''Z'' is between brackets</center>
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   <h4>[[Atomic electron configuration|Electron Configurations]]</h4>
   <h4>II. [[Atomic electron configuration|Electron Configurations]]</h4>
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The most striking similarities shared by the transition elements is  that they are all metals&mdash;which is why they are often called ''transition metals''&mdash;and that most of them are hard, strong, and shiny.  They have high melting and boiling points, and, being metals, are good conductors of heat and electricity.  Many of the elements are technologically important: [[iron]], [[nickel]], [[cobalt]], [[palladium]], [[platinum]], and others are used in heterogeneous [[catalysis]]. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance  as catalysts.
The most striking similarities shared by the transition elements is  that they are all metals&mdash;which is why they are often called ''transition metals''&mdash;and that most of them are hard, strong, and shiny.  They have high melting and boiling points, and, being metals, are good conductors of heat and electricity.  Many of the elements are technologically important: [[iron]], [[nickel]], [[cobalt]], [[palladium]], [[platinum]], and others are used in heterogeneous [[catalysis]]. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance  as catalysts.


The transition elements form many useful alloys, among themselves and with other metallic elements. Most of these elements can be dissolved in water and other polar solvents and form complexes in solution, although  the "noble" metals platinum, [[silver]], and [[gold]] are difficult to dissolve. For obvious reasons the elements [[copper]], silver, and gold are often referred to as coinage metals<ref>B. H. Lipshutz and Y. Yamamoto. ''Introduction'', Special issue of Chemical Reviews on Coinage Metals in Organic Synthesis, 2008, vol. '''108''', pp.  2793–2795 [http://dx.doi.org/10.1021/cr800415x DOI]</ref>. Note that copper belongs to the class of coinage metals, but is not a noble metal.  
The transition elements form many useful alloys, among themselves and with other metallic elements. Most of these elements can be dissolved in water and other polar solvents and form complexes in solution, although  the "noble" metals platinum, [[silver]], and [[gold]] are difficult to dissolve. For obvious reasons the elements [[copper]], silver, and gold are referred to as coinage metals<ref>B. H. Lipshutz and Y. Yamamoto. ''Introduction'', Special issue of Chemical Reviews on Coinage Metals in Organic Synthesis, 2008, vol. '''108''', pp.  2793–2795 [http://dx.doi.org/10.1021/cr800415x DOI]</ref>. Note that copper belongs to the class of coinage metals, but is not a noble metal.  
 
The outer ''s''-electrons of the transition elements are easily lost to the bonding partners (''ligands'') of the element. Also one or more ''d''-electrons are usually lost to its ligands. In other words, most transition element compounds  show ionic chemical bonds. The formal charge of the element is known as its oxidation number, or [[oxidation state]].  Table III shows the most common oxidation states of the first transition series. It is taken from reference.<ref>B. Hathaway, ''An alternative approach to the teaching of systematic transition metal chemistry'',  Journal of Chemical Education, vol. '''56''', pp. 390&ndash;392 (1979)</ref> Note in this table that the elements  exhibit variable [[oxidation state]]s.  The chemistry of the  transition series is mainly that of the ions in one of their several  oxidation states, and not that  of the elemental form itself.  For instance the element [[chromium]] (Cr) in the ionic  water complex  Cr(H<sub>2</sub>O)<sub>6</sub><sup>3+</sup> is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) The very commonly occurring  Cr(III) cation  has electronic structure [Ar](3''d'')<sup>3</sup>. The chromium in Cr(CN)<sub>6</sub><sup>4&minus;</sup> is divalent, denoted by Cr(II), and has  electronic structure [Ar](3''d'')<sup>4</sup>. Chromate [CrO<sub>4</sub>]<sup>2&minus;</sup>  contains Cr(VI), which is isoelectronic with [[argon]]. Note, parenthetically, that this classification of  transition elements in different oxidation states, although  widely and traditionally applied,  is not supported by quantum mechanical calculations. It is rare that complete  transfer of more than one full electron occurs, let alone 6 as in Cr(VI).
 
Although most theoretical descriptions of transition metal complexes assume (often implicitly) ionic bonds, quantum mechanical calculations show that  most of the bonds have a good deal of covalent character. However, in qualitative and semi-quantitative considerations, based on empirical parameters, the assumption of ionic bonds with positively charged transition metals provides much insight and yields a systematization of the properties of the transition metal complexes.
<center>
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<h3 style="height: 10px;">III. Oxidation states of first series</h3>
<font style="color: red; "> Red entries: Most common occurrence.</font> <br>
<font style="color: blue;"> Blue entries: common occurrence.</font>  <br>
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<tr align="center" style="background: silver;">
    <th> Sc
    <th> Ti
    <th> V
    <th> Cr
    <th> Mn
    <th> Fe
    <th> Co
    <th> Ni
    <th> Cu
</tr>
<tr align="center" >
    <td>  &nbsp;
    <td>  &nbsp;
    <td>  &nbsp;
    <td>  I(<i>d</i><sup>5 </sup>)
    <td>  &nbsp;
    <td>  &nbsp;
    <td>  &nbsp;
    <td>  &nbsp;
    <td>  I(<i>d</i><sup>10</sup>)
</tr>
<tr align="center" >
    <td> II(<i>d</i><sup>1 </sup>)                          <!--Sc -->
    <td> II(<i>d</i><sup>2 </sup>)                          <!--Ti -->
    <td> II(<i>d</i><sup>3 </sup>)                          <!--V  -->
    <td style = "color: blue;">  II(<i>d</i><sup>4 </sup>)    <!--Cr -->
    <td style = "color: red; ">  II(<i>d</i><sup>5 </sup>)    <!--Mn -->
    <td style = "color: red; ">  II(<i>d</i><sup>6 </sup>)    <!--Fe -->
    <td style = "color: blue;">  II(<i>d</i><sup>7 </sup>)    <!--Co -->
    <td style = "color: red; ">  II(<i>d</i><sup>8 </sup>)    <!--Ni -->
    <td style = "color: red; ">  II(<i>d</i><sup>9 </sup>)    <!--Cu -->
</tr>
 
<tr align="center" >
    <td style = "color: red; "> III(<i>d</i><sup>0 </sup>)    <!--Sc -->
    <td style = "color: blue;"> III(<i>d</i><sup>1 </sup>)    <!--Ti -->
    <td> III(<i>d</i><sup>2 </sup>)                          <!--V  -->
    <td style = "color: red; "> III(<i>d</i><sup>3 </sup>)    <!--Cr -->
    <td style = "color: blue;"> III(<i>d</i><sup>4 </sup>)    <!--Mn -->
    <td style = "color: blue;"> III(<i>d</i><sup>5 </sup>)    <!--Fe -->
    <td style = "color: blue;"> III(<i>d</i><sup>6 </sup>)    <!--Co -->
    <td> III(<i>d</i><sup>7 </sup>)                          <!--Ni -->
    <td> III(<i>d</i><sup>8 </sup>)                          <!--Cu -->
</tr>
 
<tr align="center" >
    <td>  &nbsp;                                                    <!--Sc -->
    <td style = "color: blue;"> IV(<i>d</i><sup>0</sup>)    <!--Ti -->
    <td style = "color: blue;"> IV(<i>d</i><sup>1</sup>)    <!--V  -->
    <td> IV(<i>d</i><sup>2 </sup>)    <!--Cr -->
    <td> IV(<i>d</i><sup>3 </sup>)    <!--Mn -->
    <td> IV(<i>d</i><sup>4 </sup>)    <!--Fe -->
    <td> IV(<i>d</i><sup>5 </sup>)    <!--Co -->
    <td> IV(<i>d</i><sup>6 </sup>)    <!--Ni -->
</tr>                                  <!--Cu -->
<tr align="center" >
    <td>  &nbsp;                              <!--Sc -->
    <td>  &nbsp;                              <!--Ti -->
    <td style = "color: blue;"> V(<i>d</i><sup>0 </sup>)      <!--V  -->
    <td> V(<i>d</i><sup>1 </sup>)      <!--Cr -->
    <td> V(<i>d</i><sup>2 </sup>)      <!--Mn -->
    <td> V(<i>d</i><sup>3 </sup>)      <!--Fe -->
    <td> V(<i>d</i><sup>4 </sup>)      <!--Co -->
    <td>  &nbsp;                              <!--Ni -->
    <td>  &nbsp;                              <!--Cu -->
</tr>
<tr align="center" >
    <td>  &nbsp;                              <!--Sc -->
    <td>  &nbsp;                              <!--Ti -->
    <td>  &nbsp;                              <!--V  -->
    <td style = "color: red; "> VI(<i>d</i><sup>0 </sup>)    <!--Cr -->
    <td> VI(<i>d</i><sup>1 </sup>)    <!--Mn -->
    <td> VI(<i>d</i><sup>2 </sup>)    <!--Fe -->
    <td>  &nbsp;                              <!--Co -->
    <td>  &nbsp;                              <!--Ni -->
    <td>  &nbsp;                              <!--Cu -->
</tr>
<tr align="center" >
    <td>  &nbsp;                              <!--Sc -->
    <td>  &nbsp;                              <!--Ti -->
    <td>  &nbsp;                              <!--V  -->
    <td>  &nbsp;                              <!--Cr -->
    <td style = "color: red; "> VII(<i>d</i><sup>0 </sup>)    <!--Mn -->
    <td>  &nbsp;                              <!--Fe -->
    <td>  &nbsp;                              <!--Co -->
    <td>  &nbsp;                              <!--Ni -->
    <td>  &nbsp;                              <!--Cu -->
</tr>
</table>
</center>


The elements  exhibit variable valences and form stable compounds in several formal [[oxidation state]]s (ions of certain formal charge). The chemistry of the  transition series is mainly that of the ions, in one of their several  oxidation states, and not that  of the elemental form itself.  For instance the element [[chromium]] (Cr) in the ionic  water complex  Cr(H<sub>2</sub>O)<sub>6</sub><sup>3+</sup> is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) The Cr(III) cation  has electronic structure [Ar](3''d'')<sup>2</sup>. The chromium in Cr(CN)<sub>6</sub><sup>4&minus;</sup> is divalent, denoted by Cr(II), and has  electronic structure [Ar](3''d'')<sup>4</sup>. Chromate [CrO<sub>4</sub>]<sup>2&minus;</sup>  contains Cr(VI), which is isoelectronic with [[argon]]. Note, parenthetically, that this classification of  transition elements in different oxidation states, although  still widely applied,  is not supported by quantum mechanical calculations. It is rare that complete  transfer of even one full electron occurs, let alone 6 as in Cr(VI).


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(Under construction)

A transition element is a chemical element element whose atomic electron configuration describing the ground (lowest energy) state has an incompletely filled d sub-shell. Here d stands for an atomic orbital with angular momentum quantum number ℓ = 2. That is, the electron configuration of transition element atoms contains (nd)k, 1 ≤ k ≤ 9, where n is a principal quantum number, n = 3, 4, 5, see table II. The incomplete electronic d subshell gives rise to some characteristic magnetic properties (paramagnetism and ferromagnetism) and crystals and solutions of transition metal complexes that are brightly colored.

The first three series of the transition elements are shown in table I.

I. Rows and columns of the Periodic Table of Elements containing transition elements

The atomic number Z is between brackets
Group 3 4 5 6 7 8 9 10 11
1st series Sc (21) Ti (22) V (23) Cr (24) Mn (25) Fe (26) Co (27) Ni (28) Cu (29)
2nd series Y (39) Zr (40) Nb (41) Mo (42) Tc (43) Ru (44) Rh (45) Pd (46) Ag (47)
3rd series La (57) Hf (72) Ta (73) W (74) Re (75) Os (76) Ir (77) Pt (78) Au (79)


II. Electron Configurations
Z Symbol Element Core Configuration
21Sc Scandium  [Ar](3d)1 (4s)2
22Ti Titanium  [Ar](3d)2 (4s)2
23V Vanadium  [Ar](3d)3 (4s)2
24Cr Chromium  [Ar](3d)5 (4s)1
25Mn Manganese  [Ar](3d)5 (4s)2
26Fe Iron  [Ar](3d)6 (4s)2
27Co Cobalt  [Ar](3d)7 (4s)2
28Ni Nickel  [Ar](3d)8 (4s)2
29Cu Copper  [Ar](3d)10(4s)1
39Y Yttrium  [Kr](4d)1 (5s)2
40Zr Zirconium  [Kr](4d)2 (5s)2
41Nb Niobium  [Kr](4d)4 (5s)1
42Mo Molybdenum [Kr](4d)5 (5s)1
43Tc Technetium [Kr](4d)6 (5s)1
44Ru Ruthenium  [Kr](4d)7 (5s)1
45Rh Rhodium  [Kr](4d)8 (5s)1
46Pd Palladium  [Kr](4d)10
47Ag Silver  [Kr](4d)10(5s)1
57La Lanthanum  [Xe](5d)1 (6s)2
72Hf Hafnium [Xe*](5d)2 (6s)2
73Ta Tantalum [Xe*](5d)3 (6s)2
74W Tungsten [Xe*](5d)4 (6s)2
75Re Rhenium [Xe*](5d)5 (6s)2
76Os Osmium [Xe*](5d)6 (6s)2
77Ir Iridium [Xe*](5d)7 (6s)2
78Pt Platinum [Xe*](5d)9 (6s)1
79Au Gold [Xe*](5d)10(6s)1
[Ar] stands for:   (1s)2(2s)2(2p)6 (3s)2(3p)6 [18].
[Kr] stands for:   [Ar](3d)10(4s)2(4p)6

[36].

[Xe] stands for:   [Kr](4d)10(5s)2(5p)6

[54].

[Xe*] stands for:   [Xe](4f)14 [68].


Exceptions to the definition

The elements in the fourth transition series (period 7 of the periodic table), are formally transition elements. They are man-made [except for Actinium (Z = 87)] and short-lived, not much is known about their compounds and accordingly they are not discussed in this article.

Although the atoms copper (Cu), silver (Ag), and gold (Au) have in their lowest energy state a filled d sub-shell—they have the configuration (nd)10(n+1)s1, with n = 3, 4, and 5, respectively—after ionization (loss of two or more electrons) their cations have an incomplete d sub-shell. Since these cations appear in many complexes, copper, silver, and gold are usually seen as transition elements.

In the past the group 12 elements zinc (Zn), cadmium (Cd), and mercury (Hg), that are included in the "d-block" of the periodic table, have often been considered as transition elements, but they are nowadays rarely considered as such, because their compounds lack some of their characteristic properties.

Because scandium (Sc), yttrium (Y), and lanthanum (La) actually do not form compounds analogous to those of the other transition elements and because their chemistry is quite homologous to that of the lanthanoids (previously known as lanthanide), they are often excluded from the group of transition elements. Also a strict application of the definition would describe lutetium (Lu) as a transition element as it has a singly occupied 5d orbital in its ground state, but according to IUPAC[1] it is a lanthanoid. It appears most commonly as a positive ion without d-electrons in the valence shell and without the characteristic properties of a transition element.

Properties

The most striking similarities shared by the transition elements is that they are all metals—which is why they are often called transition metals—and that most of them are hard, strong, and shiny. They have high melting and boiling points, and, being metals, are good conductors of heat and electricity. Many of the elements are technologically important: iron, nickel, cobalt, palladium, platinum, and others are used in heterogeneous catalysis. Much of the current research on the chemistry of transition element complexes is instigated by their industrial importance as catalysts.

The transition elements form many useful alloys, among themselves and with other metallic elements. Most of these elements can be dissolved in water and other polar solvents and form complexes in solution, although the "noble" metals platinum, silver, and gold are difficult to dissolve. For obvious reasons the elements copper, silver, and gold are referred to as coinage metals[2]. Note that copper belongs to the class of coinage metals, but is not a noble metal.

The outer s-electrons of the transition elements are easily lost to the bonding partners (ligands) of the element. Also one or more d-electrons are usually lost to its ligands. In other words, most transition element compounds show ionic chemical bonds. The formal charge of the element is known as its oxidation number, or oxidation state. Table III shows the most common oxidation states of the first transition series. It is taken from reference.[3] Note in this table that the elements exhibit variable oxidation states. The chemistry of the transition series is mainly that of the ions in one of their several oxidation states, and not that of the elemental form itself. For instance the element chromium (Cr) in the ionic water complex Cr(H2O)63+ is trivalent and is denoted by the oxidation state Cr(III). (This is because water has formal oxidation number zero.) The very commonly occurring Cr(III) cation has electronic structure [Ar](3d)3. The chromium in Cr(CN)64− is divalent, denoted by Cr(II), and has electronic structure [Ar](3d)4. Chromate [CrO4]2− contains Cr(VI), which is isoelectronic with argon. Note, parenthetically, that this classification of transition elements in different oxidation states, although widely and traditionally applied, is not supported by quantum mechanical calculations. It is rare that complete transfer of more than one full electron occurs, let alone 6 as in Cr(VI).

Although most theoretical descriptions of transition metal complexes assume (often implicitly) ionic bonds, quantum mechanical calculations show that most of the bonds have a good deal of covalent character. However, in qualitative and semi-quantitative considerations, based on empirical parameters, the assumption of ionic bonds with positively charged transition metals provides much insight and yields a systematization of the properties of the transition metal complexes.

III. Oxidation states of first series

Red entries: Most common occurrence.
Blue entries: common occurrence.

Sc Ti V Cr Mn Fe Co Ni Cu
      I(d5 )         I(d10)
II(d1 ) II(d2 ) II(d3 ) II(d4 ) II(d5 ) II(d6 ) II(d7 ) II(d8 ) II(d9 )
III(d0 ) III(d1 ) III(d2 ) III(d3 ) III(d4 ) III(d5 ) III(d6 ) III(d7 ) III(d8 )
  IV(d0) IV(d1) IV(d2 ) IV(d3 ) IV(d4 ) IV(d5 ) IV(d6 )
    V(d0 ) V(d1 ) V(d2 ) V(d3 ) V(d4 )    
      VI(d0 ) VI(d1 ) VI(d2 )      
        VII(d0 )        


Reference

  1. ↑ IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (online draft of an updated version of the "Red Book" IR 3-6), 2004. Retrieved on 17/9/2009.
  2. ↑ B. H. Lipshutz and Y. Yamamoto. Introduction, Special issue of Chemical Reviews on Coinage Metals in Organic Synthesis, 2008, vol. 108, pp. 2793–2795 DOI
  3. ↑ B. Hathaway, An alternative approach to the teaching of systematic transition metal chemistry, Journal of Chemical Education, vol. 56, pp. 390–392 (1979)
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