Why in any transition series, melting points first increase and then decrease and also they show a dip in the middle?

Explanation

Milting points first increase because the number of unpaired electrons increases and hence strength of metallic bond increases. After reaching the maximum, the melting points decrease because the pairing of electrons starts in the d-subshell and number of unpaired electrons decreases and so the strength of metallic bond decreases. The dip in the middle is due to exactly half-field configuration of d-subshell which has higher stability. Hence electrons are held tightly by the nucleous .As a result metallic bond is weaker.