Which of the following orders is not correct?

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(D) $1$. Heat of atomisation: $Zn, Cd, Hg$ have low enthalpies of atomisation due to their $d^{10}$ configuration. The order is $Zn > Cd > Hg$ because

Vedclass · Accepted Answer

$HgCl_2 > CdCl_2 > ZnCl_2$ (Ionic character)

Vedclass · Answer

(D) $1$. Heat of atomisation: $Zn, Cd, Hg$ have low enthalpies of atomisation due to their $d^{10}$ configuration. The order is $Zn > Cd > Hg$ because metallic bonding strength decreases down the group. This is correct.$2$. Acidic strength: Acidic character of oxides increases with the oxidation state of the metal. $Mn$ in $Mn_2O_7$ is $+7$,in $MnO_2$ is $+4$,and in $MnO$ is $+2$. Thus,$Mn_2O_7 > MnO_2 > MnO$ is correct.$3$. Oxidising nature: Oxidising power decreases down the group in the same oxidation state for $Cr, Mo, W$. Thus,$CrO_3 > MoO_3 > WO_3$ is correct.$4$. Ionic character: According to Fajan's rule,smaller cations have higher polarising power,leading to more covalent character. $Zn^{2+}$ is the smallest,so $ZnCl_2$ is the most covalent and $HgCl_2$ is the most ionic. The correct order of ionic character is $HgCl_2 > CdCl_2 > ZnCl_2$. Wait,actually,$HgCl_2$ is covalent due to high polarising power of $Hg^{2+}$. The order of ionic character is $ZnCl_2 > CdCl_2 > HgCl_2$. Therefore,the given order $HgCl_2 > CdCl_2 > ZnCl_2$ is incorrect.

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