Chemistry Form 4: Chapter 4 - Reactivity of Group 17

• When a halogen atom reacts, it gains an electron to form a singly charged negative ion which has a stable noble gas electron arrangement.
• As you go down the group of 17, the atomic radius gets bigger due to an extra filled electron shell. 
• The valence electron is further and further away from the nucleus.
• The forces of attraction between the nucleus and the valence electrons become weaker, making it more difficult to gain electrons.
• Therefore, the halogen is less reactive down the group.