The melting and boiling points first increase, reaches maximum and then steadily decrease across any transition series. ... The low melting points of Zn, Cd, and Hg are due to the absence of unpaired d-electrons in their atoms and thus low metallic bonding.

The melting point of d-block elements across a period increases to a maximum at d5 and then decreases with an increase in atomic number. This can be explained by considering the electronic configuration and the factors that affect the melting point of elements.

1. Electronic configuration:
- The d-block elements have a partially filled d-orbital in their outermost shell.
- As we move across a period from left to right, the atomic number increases, and the number of electrons in the d-orbital also increases.
- This results in an increase in the effective nuclear charge experienced by the outermost d-electrons, leading to stronger metallic bonding.

2. Factors affecting melting point:
- Metallic bonding is the primary type of bonding in d-block elements.
- Metallic bonds are formed due to the delocalization of electrons throughout the metal lattice, resulting in a sea of electrons surrounding positively charged metal ions.
- The strength of metallic bonding depends on factors such as the number of delocalized electrons, the charge on the metal ions, and the size of the metal ions.
- As the number of delocalized electrons increases from left to right across a period, the strength of metallic bonding also increases, leading to higher melting points.

3. Maximum at d5 configuration:
- At d5 configuration, the half-filled d-orbital provides maximum stability, resulting in stronger metallic bonding.
- This leads to the highest melting point among the d-block elements.
- Elements such as vanadium (V) and manganese (Mn) exhibit higher melting points due to their d5 configuration.

4. Decrease after d5 configuration:
- Beyond the d5 configuration, the number of electrons in the d-orbital continues to increase.
- The additional electrons experience increased electron-electron repulsion, which weakens the metallic bonding and lowers the melting point.
- Elements such as iron (Fe), cobalt (Co), and nickel (Ni) exhibit lower melting points compared to vanadium and manganese due to the increase in electron-electron repulsion.

In summary, the melting point of d-block elements across a period increases to a maximum at d5 configuration due to the increase in metallic bonding strength. However, beyond d5 configuration, the increase in electron-electron repulsion weakens the metallic bonding, resulting in a decrease in the melting point.