Question

can any body tell me what is the relation in between halogen group and van der wals foce of attraction.why it increase down the group in periodic table?

Answer 1

Van der waal force α area of contact.

The above mathematical relation suggests that greater the contact area, greater is the van der waal force. 

As we move down the halogen group, area of contact between the molecule increses and hence van der waal forces also increases.

Hope this cleared your doubt, feel free to ask questions and have a nice day/night ahead! :)

Answer 2

Because the size of atoms increase as you move down the periodic table.