Question

Why is the third ionisation energy of manganese (At no. 25) unexpectedly high?

Answer 1

Mn$\to {1s}^2{2s}^{2}2p^6{3s}^2{3p}^6{4s}^{2}3d^5$

$2e^{-}$ removed from ${4s}^2$. third $e^{-}$ should be removed from $3d^5$ which have stable half filled configuration (pseudo-inert gas configuration). That is why third sensation enthalpy is unexpectedly high.