Group 7 (halogens) — Edexcel IGCSE Chemistry Modular - UNIT 2 Revision Notes & Topical Questions

1. 01

   Group 7 (halogens):

   • the elements in Group 7 are known as the halogens.
   • These are fluorine, chlorine, bromine, iodine and astatine.
   • They are non-metals that are poisonous and exist as diatomic molecules held together by a single covalent bond.
2. 02

   Physical properties:

   • at room temperature fluorine and chlorine are gases, bromine is a liquid and iodine is a solid.
   • Colour darkens from pale yellow (F₂) to yellow-green (Cl₂), red-brown (Br₂) and grey-black (I₂).
   • Iodine sublimes to a purple vapour on gentle heating.
3. 03

   Why melting points rise:

   • molecules get larger down the group, so the intermolecular forces between them get stronger.
   • More energy is needed to overcome them, so melting and boiling points increase.
4. 04

   Electronic configuration:

   • every halogen atom has 7 electrons in its outer shell, e.g. Cl is 2.8.7.
   • Halogens react by gaining one electron to form a halide ion with a 1− charge and a full outer shell (Cl⁻ is 2.8.8).
5. 05

   Reactivity trend:

   • down the group the atoms have more shells, so the outer shell is further from the nucleus and more shielded by inner electrons.
   • The attraction for an incoming electron is weaker, so the atom gains an electron less easily.
   • Therefore reactivity decreases down the group.
6. 06

   Displacement reactions:

   • a more reactive halogen displaces a less reactive halogen from a solution of its halide.
   • Chlorine displaces bromine and iodine;
     • bromine displaces iodine only;
     • iodine displaces neither.
   • Colour changes show that the less reactive halogen has been released.
7. 07

   Redox in displacement:

   • in Cl₂ + 2I⁻ → 2Cl⁻ + I₂, chlorine gains electrons (is reduced;
     • it is the oxidising agent) and iodide ions lose electrons (are oxidised).
8. 08

   Hydrogen halides:

   • halogens react with hydrogen to form hydrogen halides, e.g. H₂ + Cl₂ → 2HCl.
   • HCl is a gas that dissolves in water to give hydrochloric acid, because it ionises to H⁺ ions.
   • In methylbenzene it stays as molecules, so the solution is not acidic.
9. 09

   Test for chlorine:

   • chlorine gas bleaches damp litmus paper.
   • Predictions: fluorine is the most reactive halogen;
     • astatine would be a dark solid and the least reactive.
10. 10

    Colours in solution:

    • in water, chlorine is pale green, bromine is orange (yellow-orange when dilute) and iodine is brown.
    • These colours identify the halogen released in displacement reactions.
11. 11

    Metal halides:

    • halogens react with metals to form ionic halide salts containing X⁻ ions, e.g. 2K + Br₂ → 2KBr and 2Fe + 3Cl₂ → 2FeCl₃.
    • The reactions get slower down the group.
12. 12

    Reactivity with hydrogen:

    • fluorine reacts with hydrogen explosively even in the cold and dark;
      • chlorine needs light or heating;
      • bromine and iodine react more slowly still.
    • This matches reactivity decreasing down the group.
13. 13

    Non-metal halides:

    halogens react with non-metals to form simple covalent molecules, e.g. hydrogen halides such as HCl and HBr.

Melting and boiling points increase and colours darken down the group.

A halogen displaces any halogen below it in the group.