Notes On d-Block: Oxidation States - CBSE Class 12 Chemistry
One of the most striking features of the transition elements is their ability to exhibit several oxidation states in their compounds. This unique ability is due to the small difference in energy between the ns and (n -1) d orbitals. Thus addition to ns electrons (n -1)d electrons also participate in bonding. The table here lists the common oxidation states exhibited by the first row transition elements. Sc Ti V Cr Mn Fe Co Ni Cu Zn  (+1)   +1   +2   +2   +2   +2   +2   +2   +2   +2   +2   +3   +3   +3   +3   +3   +3   +3   +3   +4   +4   (+4)   +4   (+4)   (+4)   +4   +5   (+5)   (+5)   (+5)   +6   +6   (+6)   +7 Except for scandium and zinc, all the other elements show more than one oxidation state. Scandium, due to the availability of very few electrons for bonding, does not show variable oxidation states. On the other hand, zinc, due to the presence of too many d electrons, has fewer orbitals available for bonding, and thus, does not exhibit variable oxidation states. The elements from titanium to zinc, with the exception of copper, show a minimum oxidation state of +2. Copper shows a minimum oxidation state of +1. The maximum oxidation state is given by the sum of the s and d electrons for the first five elements. Ex: scandium, titanium, vanadium, chromium and manganese exhibit maximum oxidation states of +3, +4, +5, +6 and +7 in the form of their oxides, oxyanions and oxycations. The maximum oxidation states of the elements after manganese are not at all related to their electronic configurations. Ex: Cobalt shows +2 and +3 stable oxidation states. The relative stabilities of various oxidation states of 3d series elements can be correlated with the extra stability of 3d0, 3d5 and 3d10 configurations to some extent. Thus, Ti4+ ion with 3d0 configuration is more stable than a Ti3+ ion with 3d1 configuration. The tendency to form ionic compounds decreases with an increase in the oxidation number of the metal. Higher oxidation states give rise to covalent compounds. Ex: Mn(II) oxide is ionic, Mn(IV) oxide is intermediate between ionic and covalent and Mn(VII) oxide is purely covalent. As the oxidation state of the transition metal increases, the charge density on the metal also increases. This results in an increase in the polarisation of the anion charge cloud by the metal and hence, the covalent character increases. Oxide Oxidation Number of Metal Nature of Compound    MnO    +2    Ionic    MnO2    +4    Covalent    Mn2O7    +7    Covalent The acidic nature of the oxides increases with an increase in the oxidation state of the transition element. Ex: Mn(II) oxide is basic, Mn(IV) oxide is amphoteric while Mn(VII) oxide is acidic in nature. In higher oxides, the acidic character is predominant. Thus, Mn(VII) oxide gives permanganic acid, while Cr(VI) oxide gives chromic acid and dichromic acid.

Summary

One of the most striking features of the transition elements is their ability to exhibit several oxidation states in their compounds. This unique ability is due to the small difference in energy between the ns and (n -1) d orbitals. Thus addition to ns electrons (n -1)d electrons also participate in bonding. The table here lists the common oxidation states exhibited by the first row transition elements. Sc Ti V Cr Mn Fe Co Ni Cu Zn  (+1)   +1   +2   +2   +2   +2   +2   +2   +2   +2   +2   +3   +3   +3   +3   +3   +3   +3   +3   +4   +4   (+4)   +4   (+4)   (+4)   +4   +5   (+5)   (+5)   (+5)   +6   +6   (+6)   +7 Except for scandium and zinc, all the other elements show more than one oxidation state. Scandium, due to the availability of very few electrons for bonding, does not show variable oxidation states. On the other hand, zinc, due to the presence of too many d electrons, has fewer orbitals available for bonding, and thus, does not exhibit variable oxidation states. The elements from titanium to zinc, with the exception of copper, show a minimum oxidation state of +2. Copper shows a minimum oxidation state of +1. The maximum oxidation state is given by the sum of the s and d electrons for the first five elements. Ex: scandium, titanium, vanadium, chromium and manganese exhibit maximum oxidation states of +3, +4, +5, +6 and +7 in the form of their oxides, oxyanions and oxycations. The maximum oxidation states of the elements after manganese are not at all related to their electronic configurations. Ex: Cobalt shows +2 and +3 stable oxidation states. The relative stabilities of various oxidation states of 3d series elements can be correlated with the extra stability of 3d0, 3d5 and 3d10 configurations to some extent. Thus, Ti4+ ion with 3d0 configuration is more stable than a Ti3+ ion with 3d1 configuration. The tendency to form ionic compounds decreases with an increase in the oxidation number of the metal. Higher oxidation states give rise to covalent compounds. Ex: Mn(II) oxide is ionic, Mn(IV) oxide is intermediate between ionic and covalent and Mn(VII) oxide is purely covalent. As the oxidation state of the transition metal increases, the charge density on the metal also increases. This results in an increase in the polarisation of the anion charge cloud by the metal and hence, the covalent character increases. Oxide Oxidation Number of Metal Nature of Compound    MnO    +2    Ionic    MnO2    +4    Covalent    Mn2O7    +7    Covalent The acidic nature of the oxides increases with an increase in the oxidation state of the transition element. Ex: Mn(II) oxide is basic, Mn(IV) oxide is amphoteric while Mn(VII) oxide is acidic in nature. In higher oxides, the acidic character is predominant. Thus, Mn(VII) oxide gives permanganic acid, while Cr(VI) oxide gives chromic acid and dichromic acid.

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