6.6–6.7, 6.9–6.13Group 7
The elements in Group 7 of the periodic table are known as the halogens, and have similar reactions because they all have seven electrons in their outer shell. The halogens are non-metals and consist of molecules made of pairs of atoms — Cl₂, Br₂, I₂.
In Group 7, the further down the group an element is, the higher its relative molecular mass, melting point and boiling point.
In Group 7, the reactivity of the elements decreases going down the group. Gaining one electron is what these reactions consist of, and the further down the group, the further the outer shell is from the nucleus and the more it is shielded — so an incoming electron is attracted less strongly and the element is less reactive.
You should be able to describe the nature of the compounds formed when chlorine, bromine and iodine react with metals and non-metals.
With metals, the halogens form ionic compounds — metal halides such as sodium chloride, in which the halogen has gained an electron to become a halide ion with a 1− charge.
With non-metals, the halogens form covalent compounds — molecules such as hydrogen chloride, in which pairs of electrons are shared.
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. Because reactivity decreases down the group, a halogen displaces any halogen below it and none above it.
Worked example
Chlorine solution is added to potassium bromide solution. State what you would see, and write the equation.
- Chlorine is above bromine in Group 7, so chlorine is more reactive than bromine.
- A more reactive halogen displaces a less reactive one from an aqueous solution of its salt, so chlorine displaces the bromide.
- The bromine released colours the solution orange.
- Chlorine takes the potassium; bromine is left as a molecule.
Answer: The colourless solution turns orange as bromine is displaced: Cl₂ + 2KBr → 2KCl + Br₂. Adding bromine to potassium chloride gives no reaction, because bromine is less reactive than chlorine.
Practical skill
Displacement reactions of the halogens
- Set out a grid of test tubes or a spotting tile, with a column for each halogen solution — chlorine, bromine and iodine water — and a row for each halide solution — potassium chloride, potassium bromide and potassium iodide.
- Add a few drops of each halogen solution to each halide solution.
- Record the colour of every mixture, including the ones where nothing happens.
- Compare each mixture with the colour of the halogen solution on its own, so that a colour change can be attributed to a displaced halogen rather than to the halogen added.
You should be able to explain how properties of the elements in Group 7 depend on the outer shell of electrons of the atoms, and predict properties from given trends down the group.
- Halogens
- the elements in Group 7, non-metals with seven electrons in their outer shell that exist as molecules of pairs of atoms; reactivity decreases down the group because the outer shell is further from the nucleus and more shielded, so an electron is gained less easily
6.8Test for chlorine
The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas, the litmus paper is bleached and turns white.
The paper must be damp — chlorine has to dissolve before it can bleach — so "damp" is part of the answer, not a detail of technique.
These four tests are used throughout the rest of the specification, and recognising where each one belongs is usually what a question is really asking.
Hydrogen appears whenever a metal reacts with water or with a dilute acid, and at the cathode when a solution of a reactive metal's compound is electrolysed. Oxygen appears at the anode in the electrolysis of aqueous solutions without halide ions, and when hydrogen peroxide decomposes. Carbon dioxide appears whenever a carbonate meets an acid, and when a metal carbonate is heated until it decomposes. Chlorine appears at the anode when a chloride solution is electrolysed.
Worked example
A student adds dilute hydrochloric acid to a white solid. The mixture fizzes. Describe how to find out whether the gas is hydrogen or carbon dioxide, and say what each result would mean.
- Both gases would cause fizzing, so the fizzing alone does not identify it.
- Collect the gas in a test tube.
- Test with a burning splint: if it burns rapidly with a pop, the gas is hydrogen, so the solid contained a metal.
- Otherwise bubble the gas through limewater: if the limewater turns milky, the gas is carbon dioxide, so the solid was a carbonate.
- Carry out one test per sample of gas — a burning splint would consume the gas needed for the second test.
Answer: A pop with a burning splint means hydrogen, from a metal reacting with the acid. Limewater turning milky means carbon dioxide, from a carbonate reacting with the acid.
Common mistake
Litmus paper in chlorine may first turn red, because chlorine solution is acidic, but the result that identifies chlorine is that the paper is then bleached white. An acid alone turns the paper red without bleaching it.
Before you practise
- Chlorine is a green gas, bromine an orange-brown liquid, iodine a grey solid — melting and boiling points increase down the group.
- Halogens have seven outer electrons and react by gaining one, forming 1− halide ions.
- Reactivity DECREASES down Group 7, the opposite way to Group 1.
- The reason is again the outer shell: further from the nucleus, so an electron is attracted less strongly.
- A more reactive halogen displaces a less reactive one from its salt — chlorine displaces bromine and iodine.
- Displacement is redox: the halogen gains electrons and is reduced, the halide ion loses them and is oxidised.
- Chlorine bleaches damp litmus paper white — the bleaching is the answer, not just a colour change.