Answer to Question #3897 in Inorganic Chemistry for dawood

Question

Why reactivity of halogens decreases down the group?

Answer 1

As you go down the group with increase in atomic number from one Group 7 halogen element down to the next .. F => Cl => Br => I ...
• the atomic radius gets bigger due to an extra filled electron shell,
• the outer electrons are further and further from the nucleus and are also shielded by the extra full electron shell of negative electron charge,
• therefore the outer electrons are less and less strongly attracted by the positive nucleus as would be any 'incoming' electrons to form a halide ion (or shared to form a covalent bond).
SO, this combination of factors means to attract an 8th outer electron is more and more difficult as you go down the group, so the element is less reactive as you go down the group, i.e. less 'energetically' able to form the X- halide ion with increase in atomic number.