Identify the weakest base from the following.

The electronic configuration of transition element $X$ is $[Ar] 3d^5$,and its oxidation state is $+3$. What is its atomic number?

The second ionisation energy of the following elements follows the order

Which of the following pairs does not consist of isoelectronic species? (Atomic numbers: $Sm = 62, Er = 68, Yb = 70, Lu = 71, Eu = 63, Tb = 65, Tm = 69$)

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

The correct electronic configuration of the element $Au$ is