The Halogens Explained

Chlorine keeps swimming pools safe, fluoride protects your teeth, iodine disinfects wounds and bromine helps stop furniture catching fire. Yet the same elements can be deadly poisons. The halogens are one of the most dramatic families in chemistry, and they appear in almost every IGCSE and GCSE Chemistry exam. In this guide you will learn what the halogens are, how their properties change down the group, why fluorine is the most reactive non-metal on Earth, how displacement reactions work and how to test for halide ions in the lab.

What Are the Halogens?

The word halogen comes from Greek and means "salt-former". Halogens react with metals to form salts, the most famous being sodium chloride, common table salt.

The halogens in order down the group are:

Astatine is extremely rare and radioactive. Scientists estimate there is less than a gram of it in the Earth's crust at any time, so it is rarely studied in schools.

Why Are the Halogens in the Same Group?

Every halogen atom has seven electrons in its outer shell. This is why they share such similar chemical properties. To reach a stable full outer shell, a halogen atom needs to gain just one electron. When it does, it forms a halide ion with a charge of 1−:

• Fluorine forms fluoride, F⁻

• Chlorine forms chloride, Cl⁻

• Bromine forms bromide, Br⁻

• Iodine forms iodide, I⁻

Notice how the name changes from "-ine" to "-ide" when the atom becomes an ion. This is a small detail that examiners love to test.

Halogens Are Diatomic Molecules

Halogens do not exist as single atoms. Two atoms share a pair of electrons to form a covalent bond, making a diatomic molecule: F₂, Cl₂, Br₂ and I₂. Each atom then has eight electrons in its outer shell. Because the forces between these separate molecules are weak, the halogens have fairly low melting and boiling points compared with metals. Always write chlorine gas as Cl₂ in equations, never as Cl.

Physical Properties of the Halogens and Their Trends

Trend 1: colour gets darker down the group

The colours change from pale yellow to yellow-green to red-brown to grey-black. Iodine is also famous for subliming on gentle heating, turning straight from a solid into a beautiful purple vapour.

Trend 2: melting and boiling points increase down the group

This is because the molecules get larger down the group. Bigger molecules have stronger intermolecular forces between them, so more energy is needed to separate them. That explains why fluorine and chlorine are gases, bromine is a liquid and iodine is a solid at room temperature. Using the trend, you can predict that astatine is a black solid with an even higher melting point.

Trend 3: density increases down the group

Heavier atoms packed together make denser substances.

Chemical Properties: Why Reactivity Decreases Down Group 7

This is the single most important idea in the topic and a guaranteed exam question.

When halogens react, their atoms gain an electron. The easier it is to gain that electron, the more reactive the halogen.

• Going down the group, atoms have more electron shells, so the atomic radius increases.

• The outer shell is further from the nucleus.

• There is more shielding from the inner shells.

• So the attraction between the positive nucleus and an incoming electron becomes weaker.

• It becomes harder to gain an electron, so reactivity decreases down the group.

Fluorine is therefore the most reactive halogen, and in fact the most reactive non-metal of all. It reacts with almost every element, even glass and some noble gases. Notice that this trend is the opposite of the alkali metals, where reactivity increases down the group because atoms lose electrons more easily.

Reactions of the Halogens

1. Halogens react with metals to form metal halides

Halogens react with metals to produce ionic salts called metal halides. For example, sodium burns in chlorine with a bright yellow flame to form white sodium chloride:

2Na + Cl₂ → 2NaCl

Hot iron wool glows as chlorine passes over it, forming brown iron(III) chloride:

2Fe + 3Cl₂ → 2FeCl₃

The reactions become less vigorous down the group. Iron reacts with iodine vapour only slowly, forming iron(II) iodide.

2. Halogens react with hydrogen to form hydrogen halides

Halogens react with hydrogen to make hydrogen halides, which are colourless, poisonous gases:

H₂ + Cl₂ → 2HCl

The trend in reactivity is clear here: fluorine and hydrogen explode even in the dark and cold; chlorine and hydrogen explode in sunlight; bromine needs heating and a catalyst; iodine reacts only partially, even when heated.

Hydrogen halides dissolve very well in water. Hydrogen chloride dissolves to form hydrochloric acid, a strong acid, because it releases H⁺ ions in water. Interestingly, hydrogen chloride dissolved in methylbenzene, an organic solvent, does not behave as an acid, because it stays as molecules and does not release H⁺ ions. This is a favourite A-grade exam question.

3. Halogen displacement reactions

A more reactive halogen displaces a less reactive halogen from a solution of its salt. This is how we prove the order of reactivity experimentally.

For example, when chlorine water is added to potassium bromide solution, the colourless solution turns orange because bromine is produced:

Cl₂ + 2KBr → 2KCl + Br₂

Chlorine displaces both bromine and iodine, bromine displaces only iodine, and iodine displaces nothing. This confirms the reactivity order: chlorine is more reactive than bromine, which is more reactive than iodine.

Displacement reactions are redox reactions

Displacement reactions involve both oxidation and reduction. The ionic equation for chlorine reacting with bromide ions is:

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

• Chlorine gains electrons and is reduced to chloride ions.

• Bromide ions lose electrons and are oxidised to bromine.

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons. Because chlorine causes the bromide ions to be oxidised, chlorine is acting as an oxidising agent. The potassium ions do not change, so they are spectator ions and are left out of the ionic equation.

How to Test for Chlorine Gas

Hold a piece of damp blue litmus paper in the gas. Chlorine first turns it red, because chlorine water is acidic, and then bleaches it white. Damp universal indicator paper is also bleached. Always do this in a fume cupboard because chlorine is toxic.

How to Test for Halide Ions

To identify which halide ion is in a solution:

• Add a few drops of dilute nitric acid. This removes carbonate ions, which would otherwise form a precipitate and give a false result.

• Add a few drops of silver nitrate solution.

• Observe the colour of the precipitate formed.

The ionic equation for the chloride test is: Ag⁺ + Cl⁻ → AgCl. Do not use hydrochloric acid to acidify the sample, because it would add chloride ions and ruin the test.

Uses of the Halogens in Everyday Life

• Fluorine: fluoride compounds are added to toothpaste and, in many places, to drinking water to strengthen tooth enamel. Fluorine is also used to make non-stick PTFE coatings for frying pans and to make refrigerants.

• Chlorine: kills bacteria in drinking water and swimming pools, making it one of the most important public health chemicals in history. It is used to make bleach, the plastic PVC, solvents and many medicines.

• Bromine: used in flame retardants, some pesticides, pool disinfectants and, historically, photographic film, where silver bromide darkens in light.

• Iodine: a dilute solution of iodine is an antiseptic for cleaning wounds. Iodine is also added to table salt, because a lack of iodine in the diet causes goitre, a swelling of the thyroid gland. In the lab, iodine solution is used to test for starch, turning from orange-brown to blue-black.

Hazards and Safety of the Halogens

All halogens are toxic. Chlorine was used as a chemical weapon in the First World War. Bromine is corrosive and causes painful burns, and its vapour damages the lungs. Halogen experiments should always be carried out in a fume cupboard or a well-ventilated lab, using small quantities and wearing eye protection.

Predicting the Properties of Astatine

Exams often ask you to use trends to predict the properties of an unfamiliar element. For astatine you can predict that it:

• Is a black solid at room temperature.

• Has a higher melting and boiling point than iodine.

• Is less reactive than iodine, so it would not displace iodine from iodide solutions.

• Forms astatide ions, At⁻.

• Reacts with hydrogen to form hydrogen astatide, HAt.

Common Exam Mistakes About the Halogens

• Writing chlorine as Cl instead of Cl₂ in equations.

• Saying reactivity increases down Group 7. It decreases.

• Confusing "chlorine" (the element) with "chloride" (the ion).

• Forgetting to explain reactivity using the attraction between the nucleus and the incoming electron.

• Saying chlorine "bleaches" blue litmus without mentioning the paper must be damp.

Key Takeaways

• The halogens are Group 7 non-metals with seven outer electrons that form diatomic molecules.

• Melting point, boiling point and colour intensity increase down the group.

• Reactivity decreases down the group because it becomes harder to gain an electron.

• A more reactive halogen displaces a less reactive one from its salt solution in a redox reaction.

• Silver nitrate gives white, cream and yellow precipitates with chloride, bromide and iodide ions.

The halogens are the chemical opposites of the alkali metals, and the two groups react together to form salts. Read our guide to the alkali metals next to see the other half of the story.