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.
🔬 Why Are Halogens So Reactive? (The Opposite of Alkali Metals!)
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!
Practice questions for Group 7: Halogens
How many electrons do halogens have in their outer shell?
Explain why chlorine is more reactive than bromine.