# 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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