Oxidation states:

Most of the transition elements show various oxidation states that is they exhibit variable valency in their compounds. A number of the common oxidation states of first transition series elements are described in the table below: 

Outer Ele. Confi. and O. S. for 3d- elements   

Description

The transition elements' outermost electronic configuration is (n - 1)d^1-10ns². As, the energy levels of (n-1) d and ns-orbitals are relatively close to each other; therefore both of the ns and (n-1) d-electrons are obtainable for bonding purposes. Hence, the number of oxidation states that is displayed by these elements depends on the number of d-electrons it has. An example of it is, Sc that is comprise the configuration 3d¹4s² can show an oxidation state of + 2 (only s-electrons are lost) and + 3 (while the d-electron is as well lost). The greatest oxidation state which elements of this group can exhibit can be provided through the total number of ns and (n -1) d-electrons.

The relative stability of the dissimilar oxidation states depends on the factors such as electronic configuration, lattice energies nature of bonding, stoichiometry, and the solvation energies. The greatest oxidation states are found in the fluorides and oxides since fluorine and oxygen (O₂) are the most electronegative elements. Highest oxidation state that is displayed by any of the transition metal is eight. The oxidation state of eight can be displayed by Ru and Os.

An observation of the ordinary oxidation states makes known the conclusions that are stated below:

(A) Variable oxidation states displayed by the transition elements are because of the contribution of outer ns and inner (n-1) d-electrons in bonding.

(B) In the + 2 and + 3 oxidation states the transition elements mainly make ionic bonds. In the compounds that are comprising higher oxidation states (compound made with the fluorine or oxygen), the bonds are mainly covalent. The example of it is, in permanganate ion MnO₄^-, all bonds that is formed among the manganese and oxygen are covalent.

(C) In fluorides and oxides the maximum oxidation states are observed. Highest oxidation state displayed by any transition elements (by Ru and Os) is 8.

(D) Apart from scandium, the most common oxidation state displayed by the elements of first transition series is +2. This oxidation state takes place from the loss of two 4s electrons. This depicts that after scandium, d-orbitals turn into more stable as compared to the s-orbital.

 (E) In a group, the maximum oxidation state raises along with atomic number. Like, iron shown the general oxidation state of + 2 and + 3, but ruthenium and osmium in similar group make compounds in the + 4, + 6 and + 8 oxidation states.

(F)  Transition elements such as Sc, Y, La and Ac do not demonstrate variable valency.

The bonding in the transition metals' compounds in low oxidation states is not all the time very simple.

(G) Ionisation energies and the stability of oxidation states: The values of the ionisation energies can be employed in approximation the relative stability of several transition metal compounds (or ions). Examples for this is are, Ni²⁺ compounds that are found to be thermodynamically much more stable as compared to Pt²⁺, while Pt⁴⁺ compounds are more stable than Ni⁴⁺ compounds. The relative stabilities of Ni²⁺ relative to Pt²⁺ and that of Pt⁴⁺ relative to Ni⁴⁺ can be described like this:

The four ionisation energies of Ni and Pt

So, the ionisation of Ni to Ni²⁺ needs lesser energy (2490 kJ mol^-1) than to the energy need for the production of Pt²⁺ (2660 kJ mol^-1). Hence, Ni²⁺ compounds are thermodynamically much more stable as compared to Pt²⁺ compounds.

Alternatively, formation of Pt⁴⁺ needs lesser energy (9360 kJ mol^-1) than to that needed for the formation of Ni⁴⁺(11290 kJ mol^-1). Hence, Pt⁴⁺ compounds are more stable as compared Ni⁴⁺ compounds.

This is supported by the reality that [PtCl₆]^2- complex ion is known, whereas the corresponding ion for nickel is not recognized. Although, other factors that influence the stability of a compound are as follow: i. The enthalpy of metal's sublimation. ii. Lattice and the solvation energies of the compound or ion.

(H) The transition metals as well make compounds in the low oxidation states like +1 and 0. Example for this are, nickle in, nickel tetracarbonyl, Ni(CO)₄ has zero oxidation state. Similarly Fe in posses zero oxidation state.

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