AQA Chemistry 5.1.2.6 · Combined and Triple
Group 7: the halogens
Seven outer electrons, molecules of two atoms, and a reactivity that falls down the group. Use the order to predict which halogen pushes which out of its salt.
Combined Science · Higher tier
Start here
A magnet and a paperclip.
Hold a magnet near a paperclip on the table.
If you had to guess, does it pull harder when the clip is close, or when it is far away?
What halogens are
Seven outer electrons, two atoms a molecule.
The halogens are non-metals with seven electrons in their outer shell. They exist as molecules of pairs of atoms: F2, Cl2, Br2, I2. With metals they form ionic compounds containing halide ions with a charge of −1 (sodium chloride, Na+Cl−). With non-metals they form covalent molecular compounds, such as hydrogen chloride, HCl.
H2 + Cl2 2HCl
Down the group the relative molecular mass increases, and so do the melting point and boiling point, because the intermolecular forces increase with the size of the molecules.
Astatine sits below iodine. Predict its state at room temperature.
Pick one.
The reactivity trend
Less reactive down the group.
A halogen reacts by gaining one electron. Fluorine and chlorine both have seven outer electrons; what differs is how far the outer shell is from the nucleus.
Why is fluorine more reactive than chlorine?
Pick one.
Displacement
The more reactive halogen takes the salt.
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. Predict each cell from the order chlorine, bromine, iodine; then see the colour.
chlorine water + potassium chloride
Predict.
chlorine water + potassium bromide
Predict.
chlorine water + potassium iodide
Predict.
bromine water + potassium chloride
Predict.
bromine water + potassium bromide
Predict.
bromine water + potassium iodide
Predict.
iodine water + potassium chloride
Predict.
iodine water + potassium bromide
Predict.
iodine water + potassium iodide
Predict.
The three equations to know:
Cl2 + 2NaBr 2NaCl + Br2
Cl2 + 2KI 2KCl + I2
Br2 + 2KI 2KBr + I2
Higher · ionic equations
Who takes the electrons.
Leave out the metal ions, which do not change. The halogen molecule gains electrons (reduction); the halide ions lose them (oxidation).
Cl2 + 2Br− 2Cl− + Br2
Chlorine is reduced: each Cl2 gains two electrons to make 2Cl−. Bromide is oxidised: each Br− loses one electron. Charges balance: 2− on each side.
In Br2 + 2I− 2Br− + I2, which species is oxidised?
Pick one.
Oxidation and reduction in terms of electrons: Oxidation and reduction.
Think again · which way is the trend?
“Like Group 1, the halogens get more reactive going down the group.”
Your call
ChooseWhich halogen is the most reactive, and why?
Sort
Reacts or no reaction?
Use the order chlorine, bromine, iodine.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Predict
- Whether a displacement happens; astatine’s properties from the trends.
- Explain
- Why reactivity falls down the group, in terms of the outer shell and the nucleus.
- Describe
- The compounds halogens form with metals and with non-metals.
Key fact
Halogens gain one electron. Down the group the outer shell is further from the nucleus, so the attraction for an incoming electron is weaker and reactivity decreases. A more reactive halogen displaces a less reactive one from its salt solution.
Key note · AQA 5.1.2.6 · Combined and Triple
Group 7: the halogens
- Group 7 elements are non-metals with seven electrons in their outer shell. They consist of molecules made of pairs of atoms (F2, Cl2, Br2, I2).
- With metals they form ionic compounds in which the halide ion carries a charge of −1; with non-metals they form covalent molecular compounds (H2 + Cl2 → 2HCl).
- Down the group relative molecular mass, melting point and boiling point increase: larger molecules, stronger intermolecular forces. Boiling points: F2 −188, Cl2 −34, Br2 59, I2 184 °C.
- Reactivity decreases down the group: the outer shell is further from the nucleus, so an incoming electron is attracted less strongly. A more reactive halogen displaces a less reactive one from an aqueous solution of its salt: Cl2 + 2NaBr → 2NaCl + Br2.
- Higher: ionic equation Cl2 + 2Br⁻ → 2Cl⁻ + Br2. Chlorine gains electrons (reduced); bromide loses electrons (oxidised).
Photograph this card. Tomorrow, cover it and say each line first.
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Mixing up the Group 1 and Group 7 trends?
Boiling points: fluorine −188 °C, chlorine −34 °C, bromine 59 °C, iodine 184 °C.