why boiling point inc from top to bottom in 15th group

Different groups exhibit different trends in boiling and melting points.

For groups 1 and 2 -> The boiling and melting points decrease as you move down the group.

For the transition metals -> Boiling and melting points mostly increase as you move down the group, but they increase for the zinc family.

The boron and carbon families (groups 13 and 14) -> Decrease in their boiling and melting points as you move down the group,

Whereas the nitrogen, oxygen, and fluorine families (groups 15, 16, and 17) tend to increase in both.

The noble gases (group 18) decrease in their boiling and melting points down the group.

REASON : These phenomena (variation in BP and MP) can be understood in relation to the types of forces holding the elements together. For metallic species, the metallic bonding interaction (electron-sharing) becomes more difficult as the elements get larger (toward the bottom of the table), causing the forces holding them together to become weaker.

As you move right along the table, however, polarizability and van der Waals interactions predominate, and as larger atoms are more polarizable, they tend to exhibit stronger intermolecular forces and therefore higher melting and boiling points.