Why Reactivity Decreases Down Group 7

⚙️ Why Reactivity Decreases Down Group 7

Halogens react by gaining one electron to achieve a full outer shell. The ease with which they do this depends on how strongly the incoming electron is attracted towards the outer shell. In fluorine, the outer shell (second shell) is very close to the positively charged nucleus — the electrostatic attraction is very strong, so fluorine pulls an electron towards itself with great force. It is the most reactive halogen. In chlorine, the outer shell is the third shell, further away — the attraction is weaker. In bromine (fourth shell) and iodine (fifth shell), the outer shell is progressively further from the nucleus, and there are more inner electron shells providing shielding. This shielding further reduces the effective nuclear charge felt at the outer shell. The result is that iodine can barely attract additional electrons at all compared to fluorine. This is the direct opposite of Group 1, where bigger atoms are MORE reactive because they lose electrons more easily rather than gaining them.