Group 7: the halogensIB MYP Chemistry: Revision notes
Section 1
What are the halogens?
Group 7 contains fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). They are called the halogens. Each atom has seven electrons in its outer shell.
The halogens exist as diatomic molecules, which means two atoms joined by a covalent bond: F₂, Cl₂, Br₂ and I₂.
When a halogen atom gains one electron it forms a halide ion with a charge of 1−, such as Cl⁻ or I⁻.
Section 2
Colour and state at room temperature
- Fluorine F₂: pale yellow gas
- Chlorine Cl₂: pale green gas
- Bromine Br₂: red-brown liquid (gives off an orange vapour)
- Iodine I₂: grey-black solid
The colours get darker down the group.
Learn them in order: gas, gas, liquid, solid, from fluorine down to iodine.
Section 3
Trend in melting and boiling points
Melting points and boiling points increase down the group. That is why chlorine is a gas but iodine is a solid at room temperature.
For example, the boiling point of chlorine is −34 °C but iodine boils at 184 °C. You can use the trend to predict the properties of other halogens: fluorine, above chlorine, has even lower melting and boiling points.
Section 4
Reactivity decreases down the group
Halogens react by gaining one electron into the outer shell. Going down the group:
- the atoms have more shells, so the outer shell is further from the nucleus
- there is more shielding
- so the nucleus attracts an extra electron less strongly
- the atom gains an electron less easily, so reactivity decreases
Fluorine is the most reactive halogen and iodine the least reactive of these four.
This is the opposite of Groups 1 and 2. Group 7 reactivity decreases down the group, because atoms are gaining an electron, not losing one.
Section 5
Displacement reactions
A more reactive halogen displaces a less reactive halogen from a solution of its halide.
- chlorine + potassium bromide → potassium chloride + bromine: Cl₂ + 2KBr → 2KCl + Br₂ (solution turns orange)
- chlorine + potassium iodide → potassium chloride + iodine: Cl₂ + 2KI → 2KCl + I₂ (solution turns brown)
- bromine + potassium iodide → potassium bromide + iodine: Br₂ + 2KI → 2KBr + I₂ (solution turns brown)
A less reactive halogen cannot displace a more reactive one, so iodine with potassium bromide gives no reaction.
Ask: is the halogen being added more reactive than the halide's halogen? If yes there is a reaction; if no, nothing happens.
Section 6
Uses of halogens
- Chlorine is added to drinking water and swimming pools to kill bacteria and make the water safe. Chlorine is toxic, so it must be used in small, controlled amounts.
- Iodine solution is used as an antiseptic to clean skin and wounds before surgery.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Group 7: the halogens
- A nurse in a clinic in Nairobi cleans a patient's cut with a brown iodine solution before applying a dressing. Iodine is a Group 7 element, one of the halogens.Explain why the nurse uses iodine solution on the cut.2 marks
- A chemist adds a few drops of orange bromine water to a colourless solution of potassium iodide, and the mixture turns brown. She then adds bromine water to a colourless solution of potassium chloride and sees no change.Write the balanced symbol equation for the reaction of bromine with potassium iodide.2 marks
- A data book gives these melting and boiling points. Chlorine melts at −101 °C and boils at −34 °C. Bromine melts at −7 °C and boils at 59 °C. Iodine melts at 114 °C and boils at 184 °C. A chemist wants to predict the properties of fluorine, which lies above chlorine in Group 7. Room temperature is 25 °C.State the physical state of chlorine, bromine and iodine at 25 °C.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).