why in transition elements ,the melting point first increases and then decreases

Dear Student, The strength of interatomic bonds in transition elements depend upon the number of unpaired electrons in d-orbital. In beginning of the series number of unpaired electrons keep on increasing till the middle causing increase in interatomic bond and so melting point also increases. But after it pairing of electron take place and so number of unapaired electron decreases leading to decrease in interatomic bonding and therefore decreasing melting point. Regards,