Explaining the Reactivity Trend of the Halogens

1. The reactiveness of halogens decreases down the group.
2. This can be explained as below:
1. When a halogen atom reacts, it gains an electron to form a singly negative charged ion.
2. As we go down the group from F => Cl => Br => I, the size of the atom increases due to an extra filled electron shell.
3. The valence electrons are further and further from the nucleus, the attraction force between the electrons and the nucleus become weaker and weaker.
4. Therefore the ability of the atom to attract electron to fill the outermost shell reduces., which means the reactiveness of the atom reduces.