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How do the oxidation states of the elements vary in the transition series? |
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Answer» Solution :In the beginning of the series, very less number of d-electrons are available for the chemical bonding. Hence, less number of oxidation states are shown by elements present at the beginning of series. Ex: Scandium `(d^(1))` is known only in (+3). Titanium `(d^(2))` exists in (+2), (+3) and (+4). However, Ti(IV) is more stable than Ti(III) or Ti(II). At the end of the series, there are too many d-electrons and d-orbitals are completely occupied. Ex: Zn and Cu hence, there are very less number of orbitals available to share electrons with others for higher valance. Hence, these elements show very less number of oxidation states. Ex: Zn(II) is only known while copper is known to exist in Cu(I) or Cu(II). The greatest number of oxidation states of the elements are known in the middle of the series. Ex: Mn shows oxidation state from (+2) to (+7). The stability of the higher oxidation states corresponds in value to the SUM of s and d-electrons upto manganese adn then decreases abruptly. Ex `Ti^(IV) O_(2), V^(V) O_(2_(+)), Cr_(VI) O_(4)^(2-), Mn^(VII) O_(4)^(-), Fe^(II, III), Co^(II, III), Ni^(II), Cu^(I, II), Zn^(II)` The ELMENTS showing more than one oxidation states have the difference on oxidation state of unity. This is opposite to the oxidation states shown by non-transition elements which normally differs by two. Ex: `V^(II), V^(III), V^(IV), V^(V)`. Down the group, the stability of elements in higher oxidation states increases because the removal of electrons from d-orbitals becomes easy. Ex: Mo(VI) and W(VI) are found to be more stable than Cr(VI). As a result, Cr(VI) in the form of dichromate act as strong oxidizing agent in acidic medium where as `MoO_(3) and WO_(3)` are not. Thus in p-block elements, the lower oxidation states is favoured by higher members of the group due to inner PAIR effect while in transiition elements from group -4 to group -10, higher members show high oxidation state. Low oxidation states are found when a complex compound has ligands capable of `PI`-acceptor character in addition to `sigma`-bonding Ex: In `Ni(CO)_(4) and Fe(CO)_(5)`, the oxidation state of nickel and iron is zero. |
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