Halogens
Group 17 non-metals form salts and are strong oxidisers.
About this topic
The halogens (group 17: F, Cl, Br, I, At) have seven valence electrons and are highly reactive non-metals, gaining one electron to form −1 ions called halides. They exist as diatomic molecules, and go from pale yellow gas (fluorine) and green gas (chlorine) to a red-brown liquid (bromine) and a grey-black solid (iodine) down the group.
Reactivity decreases down the group, the opposite of the alkali metals, because the incoming electron is further from the nucleus and more shielded. A more reactive halogen displaces a less reactive one from its salt solution: chlorine added to potassium bromide makes bromine, shown by an orange colour. The equation is Cl2 + 2KBr gives 2KCl + Br2. Halogens react with metals to produce salts, the name meaning salt-formers.
Chlorine disinfects drinking water and swimming pools, fluoride in toothpaste strengthens enamel, iodine is an antiseptic and is needed by the thyroid, and bromine compounds are used in flame retardants. Chlorofluorocarbons damaged the ozone layer before being banned.
Students often think that reactivity goes up the way it does in group 1; remember the reverse trend. Also, halogens are toxic as elements but halide ions, like chloride in salt, are harmless and needed by the body.
Key ideas
- Seven valence electrons
- Reactivity decreases down the group
- Form 1− ions
Quick check
Use cases: where Halogens helps
- Homework: review the key idea behind Halogens before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Halogens without notes.
- Visual learning: play with the interactive lab to see how Halogens changes when you adjust its inputs.
- Teaching a friend: explain Halogens in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Halogens?
Group 17 non-metals form salts and are strong oxidisers.
What will I learn in Halogens?
Seven valence electrons. Reactivity decreases down the group. Form 1− ions.
Is Halogens part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Halogens lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Halogens?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.