Variable Electrovalency

The elements which change their valency or show more than one electrovalency

are said to possess variable electrovalency. This is due to following two reasons.


(i) Unstable configuration of the core: since in transition metals, core possesses

unstable arrangement of electrons (ns^2 np^6 nd^{1- 9}) hence they show variable

valency. For example, Fe^{++} (3s^2 3p^6 3d^6) lose an electron to change into

Fe^{++} (3s^2 3p^6 3d^5).

Now due to increase of effective nuclear change Fe^{3+} do not lose more electron.

Similarly, we can explain the formation of Cu^+ \text{and} Cu^{++}; Co^{2+} \text{and } Co^{3+} ions etc.


(ii) Inert pair effect: it is observed that in the case of ns^2 np^{1- 5}

configuration, ns^2 electrons have extra stability and hence reluctant to take part in bond formation.

The reluctance of s-electron pair to take part in bond formation is known as

inert pair effect. This effect increases as atomic number increases in the group.

This is the reason that in  14th group of the periodic table the compounds of Ge^{2+} are unstable while those of Pb^{2+} are stable. Thus Ge^{2+}  ions oxidize to Ge^{4+}

ions more easily than Pb^{2+} to Pb^{4+}.

Therefore Pb^{4+} compounds are powerful oxidizing agent and Ge^{2+} compounds are reducing agents.

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