chem 既問題...............................

因為halogen molecules之間以intermolecular force/van der Waals' force拉著
這股force會除著molecular size增加而增加 -> boiling point增高
而halogen的molecular size正是increase down the group
所以intermolecular force (hence boiling pt) 也是increase down the point

2007-12-26 23:51:55 補充：
Last line last word "point" - should be "group"

Halogens always exist in pairs. i.e.

Cl2, Br2, etc.

The size of the nucleus increases down the group. Therefore the bonding force between two atoms (van der waal force) increases. That's why boiling point of halogens increases down the group.