Metal of transition

The 38 chemical elements of Atomic number 21 to 30,39 to 48,72 to 80 and 102 to 110 are commonly called the metals of transition . This name comes from their position in the periodic Tableau of the elements, which represents the successive addition of a electron in the orbital '' D '' of the Atome S when one passes from the one to the other through the period. Metals of transition are strictly defined by IUPAC as “the elements which have in an atomic state an underlayer D partially filled or which forms at least an ion with an underlayer D partially filled”, which excludes the elements from group 12 (Zn, Cd, Hg) which have an electronic configuration d¹⁰ in an atomic state and in their stable state ionic which is discharged. A less strict definition is usually adopted in which the term transition refers in the passing between group 2 (in the past IIA) and groups it 13 (in the past IIIA) periodic table.

Electronic configuration

The elements coming before the group from metals from transition (elements 1-20) in the periodic table do not have an electron in orbital the D , but only in orbital the S and p .

Scandium with the Zinc, the elements of the block D gradually fill their orbital D through the series. Except for Copper and Chromium, all the elements of the block D have two electrons in their external underlayer N S , even the elements with an underlayer (n-1) D incomplete. This is unusual because normally the orbital ones of principal quantum Nombre weaker are filled before the orbital ones of principal quantum Nombre higher, but in this case orbital N S of the elements of the block D have an energy weaker than orbital (n-1) the D . As the atoms are always in their energy state most stable, orbital N S are filled initially (see the article Diagramme of Klechkowski). Copper and chromium have only one electron in their orbital external 4 S because the fact of having a layer 3 D completely filled (Cu) or half-filled (Cr) brings a particular electronic stabilization.

See also: electronic Configuration

Chemical properties

The transition metals have in general strong a Densité, a Melting point and high vaporization (except the mercury which is liquid with room temperature). These properties come from the capacity of the electrons of the underlayer D to be delocalized in the metal network. In the metal substances, plus the number of electrons divided between the cores is large, larger is the cohesion of metal.

Here four characteristics spread in the family of metals of transition:

they form coloured compounds;
they have many states of oxidation;
certain elements are good Catalyseur S;
they can form complex .

Variable states of oxidation

Compared with metals Alkaline-earth (group 2) such as the Calcium, metals of transition, except those of groups 3 and 12, can form ions with a large variety of states of oxidation. Alkaline-earth metals, such as calcium are stable with the state of oxidation +II, whereas a metal of transition can adopt oxidation steps of - III with +VIII. One can include/understand the reason of it by studying the potentials of ionization of the two groups. Necessary energy to remove an electron of calcium is weak until one starts to remove from the electrons under the two electrons of his underlayer 4 S . In fact Ca³⁺ has such an energy of ionization that it naturally is not found. On the other hand with an element as the Vanadium one observes a linear increase in the energy of ionization between orbital the S and D , this being with the very weak energy difference between orbital the 3D and 4s.

Certain reasons can be observed through one period of metals of transition:

the number of states of oxidation of each ion increases to manganese, then decreases. This fall is caused by strongest attraction between the protons of the core and the electrons, making them more difficult successive ionizations.
When an element is in a weak state of oxidation, one can find it in the form of simple ion. On the other hand for the highest states of oxidation one often finds it in the form of compound related in a covalent way on the Oxygène or the Fluor.

Catalytic activity

Certain metals of transition form good homogeneous and heterogeneous catalysts. For example, the Fer is a catalyst in the Procédé of Haber, the Nickel and it platinum are used in the Hydrogénation Alcènes.

Composed coloured

Because of their electronic structure, metals of transition form many ions and coloured complexes. Their color can even vary according to the states of oxidation of the same ion. For example MnO₄^- (mn in a state of oxidation +7) is purple, whereas Mn²⁺ is pink pale.

The formation of complexes can play a big role in the color of a compound having a metal of transition. This comes owing to the fact that the ligand S have an effect on underlayer 3 D . As the formed complexes have a symmetry lower than the speric symmetry of the naked metal ion, the energy levels of orbital the 3D are not any more the same ones. There can thus be electronic transitions between orbital the 3 D , which are often in the visible radiation. One can thus observe a modification of the color of the ion complexed according to the ligands used.

The color of a complex depends on:

the nature of the metal ion, particularly of the number of electrons in its orbital D ;
of the arrangement of the ligands of metal, for example of the Isomère S can have a different color;
the nature of the ligands surrounding the metal cation.

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