Every halogen has 7 electrons in its outer shell — just ONE short of a full shell.
Think of it this way:
- Atoms "want" full outer shells (8 electrons)
- Halogens only need to gain 1 electron to be full
- So they're desperate to grab an electron from anything nearby!
Why reactivity DECREASES down the group:
- Going down, atoms get bigger (more shells)
- The outer shell is further from the positive nucleus
- There's less attraction to pull in a new electron
- So it's harder to gain an electron = less reactive
OPPOSITE to Group 1! In Group 1, reactivity INCREASES down. In Group 7, reactivity DECREASES down. Make sure you know why!
This deep dive covers Why Are Halogens So Reactive? (The Opposite of Alkali Metals!) within Group 7: Halogens for GCSE Chemistry. Revise Group 7: Halogens in Atomic Structure for GCSE Chemistry with 22 exam-style questions and 21 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 3 of 12 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Group 7: Halogens
How many electrons do halogens have in their outer shell?
Explain why chlorine is more reactive than bromine.