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.

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 Virtually all metals have either a close-packed (hcp or ccp) or a body-centered (bcc) structure, and all the TM (including Tc and Re in the same group as Mn) adopt these structures EXCEPT Mn! 


It has a bizarre structure. Now I haven't been following the lit but certainly ten years ago, "there has been no generally accepted interpretation of the structure." 
A. F. Wells, Structural Inorganic Chemistry 5th ed (1993) p 1306
Some weird quantum mechanical effect arising from the [Ar]3d^5 4s^2 electronic configuration? Don't forget that ferromagnetism chips in at Fe.
You can see if Wikipedia has more recent info.
Because the Mn atoms are not close packed (or nearly so as in bcc) the lattice is not as strong as Cr (bcc) and hence Mn has a lower mp than Cr.
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 The atomic no.of chromium is 24 whereas the atomic no. of manganese is 25.

Size decreases along the period therefor manganese is smaller than chromium.
hence smaller atoms have low melting and boiling point.
 
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