Compare the stability of the $+2$ oxidation state for the elements of the first transition series.

(N/A) The stability of the $+2$ oxidation state in the first transition series ($3d$ series) generally increases as we move from left to right across the period.
This trend is observed because the sum of the first and second ionization enthalpies increases across the series,making the removal of the third electron from the $d$-orbital increasingly difficult.
Specifically,for elements like $Mn^{2+}$ ($d^5$ configuration) and $Zn^{2+}$ ($d^{10}$ configuration),the $+2$ state is particularly stable due to the half-filled and fully-filled $d$-orbital configurations,respectively.
Conversely,for elements on the left side of the series,higher oxidation states are often more stable because it is easier to remove electrons from the $d$-orbitals.