Variable valency is shown by

Which element from the following has the lowest first ionization enthalpy $(IE_1)$?

Identify the order in which the spin-only magnetic moment (in $BM$) increases for the following four ions:
$(I) Fe^{2+}$
$(II) Ti^{2+}$
$(III) Cu^{2+}$
$(IV) V^{2+}$

Correct statement for $3d-$ series is

$A$ copper salt is isomorphic with $ZnSO_4$. The salt will be:

Why are $Mn^{2+}$ compounds more stable than $Fe^{2+}$ towards oxidation to their $+3$ state?