The correct order of $E^{\circ}_{M^{2+}/M}$ values for the four successive elements $Cr$,$Mn$,$Fe$,and $Co$ is:

Name the periodic element,which does not show variable oxidation states.

Which of the following transition metals has the lowest density?

Why do the transition elements exhibit variable oxidation states?

Which of the following statements are correct about $Zn$,$Cd$ and $Hg$?
$(I)$ They exhibit high enthalpies of atomisation as the $d-$subshell is full.
$(II)$ $Zn$ and $Cd$ do not show variable oxidation states while $Hg$ shows $+I$ and $+II$.
$(III)$ Compounds of $Zn$,$Cd$ and $Hg$ are paramagnetic in nature.
$(IV)$ $Zn$,$Cd$ and $Hg$ are called soft metals.

Out of $Cu_{2}Cl_{2}$ and $CuCl_{2}$,which is more stable and why?