Transition elements possess high melting and boiling points. This is due to the involvement of electrons from both $\mathrm{ns}$ and $(\mathrm{n}-1)\mathrm{d}$subshells in metallic bonding.

The physical characteristics of transition metals are explained by the solid-state band theory, which also applies to metals.

Simply said, the valence shell, or $\mathrm{ns}$, and penultimate shell, or $(\mathrm{n}-1)\mathrm{d}$ subshell, of very closely spaced atoms (very vast in number), overlap to produce overlapping bands (molecular orbital theory) and they have strong bonds due to delocalised electrons in the outermost shell of an atom.

Therefore, Transition metals have high melting and boiling temperatures, high heat of atomization, and other properties.