Q- why the melting point of manganese is less than chromium if their d orbital is halffilled
Dear student!
The melting point increases with increase in maximum no. of unpaired electrons which is genuinely found in Cr (3d54s1 ) but, after chromium, the pairing starts which decreases the melting point of manganese as Mn (3d54s2 ).
So, after Cr, the melting point starts to decrease.