Why the transition elements are hard and have high melting and boiling points?

In a transition elements, the strength of the bonds depend on the number of unpaired electrons. Greater the number of unpaired electron in $(n-1) d$ subshell, stronger are the metallic bondings and greater is the bond strength. This increases the hardness of the metal.

The high melting points of these elements is due to strong metallic bonds between the atoms that shows covalent character. Hence, large amount of energy is required to break the bonds to melt the metals.

In a period, the melting points of a transition element increases with the increase in number of unpaired electrons in (n-1)d subshell. As a result in first transition series, chromium $\left(\mathrm{d}^{5}\right)$ have maximum melting point and is typically hard metal. The more number of unpaired electrons in chromium is responsible for these properties.

After chromium the electrons start getting paired up which results in decrease in strength of bonds. Hence, after the middle of the series, the melting points start decreasing.

$\mathrm{Zn}, \mathrm{Cd}$ and $\mathrm{Hg}$ are soft and volatile because these elements have no unpaired electrons. Hence, they have low melting points.