Question

Why the boiling point of bismuth is less than that of antimony, while in tha case of other elements of thia group, it does not happen ?

Answer 1

The boiling point of bismuth is lower than antimony because it tend to form three covalent bonds instead of five due to inert pair effect.
Inert pair effect: the tendency of the electrons in the outermost s-orbital to remain unionized or unshared.
Consequently, the attraction between the atoms of bismuth is weak, leading to lowered boiling point.