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

Explanation

Melting points first increase because the number of unpaired electrons increasesand hence strength of metallic bondincreases. After reaching themaximum, the melting points decrease the pairing of electrons starts in the d-subshell and number of unpaired electrons decreases and so the strength of metallic bond decreases. Thedip in teh middle is due to exactly half-filled configuration of d-subshell which has higher stability . Hence, electrons are held tightly by the nucleus. As a result , metallic bond is weaker.