The boiling point of the halogen elements increase down the group from chlorine to bromine to iodine. Please explain this trend for 3 marks.

The number of electrons in the halogen atom increases as you move down the group (1st mark achieved here), causing stronger London forces to arise between the molecules of said halogen (2nd mark achieved here). The stronger London forces require more energy to overcome (3rd mark achieved here) leading to the increased boiling point of the halogen element as you move down the group.