The highest oxidation state of $Cr$ is:

Cuprous ion $(Cu^+)$ is colourless while cupric ion $(Cu^{2+})$ is coloured because

In the periodic table,the first transition series is placed in:

Ferrous ion changes to $X$ ion upon reacting with acidified hydrogen peroxide. The number of $d$-electrons present in $X$ and its magnetic moment (in $BM$) are,respectively:

The second ionisation energy of the following elements follows the order

In the following pairs,the one in which both transition metal ions are colourless is