What are Transition Elements?
The d-block elements are called transition elements. These are the elements that have incompletely filled (partly filled) d-subshells in their ground state or in any one of their oxidation states. Here, we will discuss the trends in the stability of oxidation states of transition metals.
The transition metals exhibit a variable number of oxidation states in their compounds. This is one of the notable features of the transition elements. Few elements show exceptions for this case, most of these show variable oxidation states. These different oxidation states are relatable to the electronic configuration of their atoms. The stability of oxidation states for transition metals depends on the electronic configuration of the elements. For example, oxidation states of transition metals exhibited by elements of the first series are listed in the table given below.
Oxidation State of Transition Elements
The existence of the transition elements in different oxidation states means that their atoms can lose a different number of electrons. This is due to the participation/contribution of inner (n-1) d-electrons in addition to outer ns-electrons because the energies of the ns and (n-1)d-subshells are almost equal. For example, scandium has the electronic configuration 3d14s2. The oxidation state of sc is +2 when it uses both of its 4s- electrons for bonding. It can also show the oxidation state of +3 when it uses its two s- electrons and one d- electron.
The other atom similarly shows oxidation states equal to ns and (n-1) d- electrons. The oxidation state of zn is +2. The chromium shows variable oxidation states; +2, +3, +4, +5, +6. The highest oxidation state of chromium is +6.
As we know the oxidation state of Zn is +2. It does not show a variable oxidation state. Therefore, does not consider a transition element.
Variable Oxidation State of Metals
The elements of the second and third transition metals series also exhibit variable oxidation states are given below:
Second Transition Series
Third Transition Series
It can be noted from the above tables that the stability of a given oxidation state is dependent upon the nature of the elements with which the metal is combined. The highest oxidation states are shown in the compounds of fluorides and oxides because fluorine and oxygen are the most electronegative elements.
The elements that show the greatest (high) number of oxidation states occur in or near the middle of the series. For example, in the first series of the d-block elements, manganese exhibits all the oxidation states from +2 to +7. The small number of oxidation states at the extreme left-hand side end is due to the lesser number of electrons to lose or share. On the other hand, at the extreme right-hand side end, it is due to a large number of d- electrons so that only fewer orbitals are available in which the electron can share with other compounds for higher valence.
In the +2 oxidation state (O.S) and +3 oxidation state, the bonds formed are mostly ionic. In the compounds or molecules of higher oxidation states (generally formed with oxygen and fluorine), the bonds are essentially covalent. Therefore, the bonds in +2 O.S and +3 O.S are generally formed by the loss of two or three electrons, while the bonds in higher oxidation states are formed by sharing of d- electrons.
Did You Know?
The variable oxidation state of transition elements is due to the participation or contribution of inner (n-1) d and outer ns- electrons.
The lowest oxidation state corresponds to the number of ns orbital electrons.
Except for scandium, the most common oxidation state of the first-row transition elements is +2 which arises due to the loss of two 4s- electrons. This means that after scandium 3d block orbitals become more stable and, therefore, are lower in energy than the 4s-orbitals. As a result, electrons are first removed from 4s- orbitals.