The chloride of which of the following elements will be coloured?

For the four consecutive transition elements $Cr, Mn, Fe,$ and $Co$,the correct order of stability of their $+2$ oxidation state is:

Why are the $E^o$ values for $Mn^{2+}/Mn$,$Zn^{2+}/Zn$,and $Ni^{2+}/Ni$ more negative than expected? The table below provides the values:
| $E^o_{Mn^{2+}/Mn}$ | $E^o_{Zn^{2+}/Zn}$ | $E^o_{Ni^{2+}/Ni}$ |
| :--- | :--- | :--- |
| $-1.18 \ V$ | $-0.76 \ V$ | $-0.25 \ V$ |

The second ionisation energy of the following elements follows the order

Explain on what factors the ionization enthalpies of elements having $d^{n}$ configuration depend upon? Explain the variation in ionization enthalpies of transition elements in the $3d$ series.

What is the atomic number of an element having $ns^1$ electronic configuration and belonging to the $3d$ transition series?