CHEM C11: Group 17 Elements

Today we’re covering:

  • Introduction to Group 17
  • Properties of Halogens
    • Physical properties of halogens
    • Chemical Properties of halogens
      • Reactivity as oxidising agents
      • Displacement reactions
      • Reactions with hydrogen

Let’s go!

Introduction to Group 17
Group 17 elements are referred to as HALOGENS.

Periodic table
From Xactly

Characteristics of Group 17:

  • Part of the p-block:
    Outer electron occupies a p-subshell
  • 7 electrons in the outermost principal quantum shell:
    2 in an s-subshell, 5 in the p-subshell
  • Non-metals:
    Covalent bonding, low melting point, non-conductive etc
  • Form ANIONS:
    Gains electrons

Trends in Properties of Group 17 Elements
Trends going down the group are explained by:

  • Higher number of shells
  • Atomic radius increases
  • Attraction between nucleus & valence electrons gets weaker
Property Going Down Group: Why?
Colour Becomes darker Not in syllabus
Atomic Radius Increases Higher number of shells
Ionic Radius Increases Ions are larger than corresponding atoms (more outer shell electrons cause larger interelectronic repulsion)

Going down: higher number of inner shells

Melting Point

Boiling Point/

Increases Intermolecular forces are van der waals forces of attraction (temporary dipole-dipole attraction)

Going down: higher number of electrons, stronger dipole-dipole forces of attraction

Electronegativity Decreases
  • Atomic radius increases
  • Weaker attraction between nucleus & outer electrons
  • Less attraction for additional electrons
Electron Affinity Decreases
(less negative)
  • Atomic radius increases
  • Weaker attraction between nucleus & outer electrons
  • Less energy released when an electron is added to form anion

(as Oxidising Agents)

  • Atomic radius increases
  • Weaker attraction between nucleus & outer electrons
  • Less tendency to accept electrons
  • Less strength in oxidising other species


General Reactions of Halogens

Reacts with metals Halogens accept electrons & become halide ions 2Na + Cl2 → 2M+ + 2Cl
Displacement Reactions between halide salts & halogens A more reactive halogen can displace a less reactive halogen from a halide solution of the less reactive hydrogen.

  • More reactive = more electronegative
  • Stronger tendency to form anions

In experiments:

  • Colour change occurs
    • Halide ions are colourless
    • Halogens are coloured
  • Thus, solution will change colour to the displaced halogen
  • Colours can be distinguished by adding cyclohexane
  • Halogen dissolves in cyclohexane layer above water
Chlorine is more reactive than bromine:

Cl2 + 2Br → 2Cl + Br2


Br2 + 2Cl

React with non-metals to form covalent compounds Halogen atom shares 1 pair of electrons with other species H2 + Cl2 → 2HCl


Reactions of Halogens with Hydrogen Gas

General Reaction X2 (g) + H2 (g) → 2HX (g) Forms hydrogen halide gas
F F2 (g) + H2 (g) → 2HF (g) Reacts explosively  (even in cold/dark conditions)
Cl Cl2 (g) + H2 (g) → 2HCl (g) Reacts explosively (in sunlight)
Br Br2 (g) + H2 (g) → 2HBr (g) Reacts slowly on heating
I I2 (g) + H2 (g) ⇌ 2HI (g) Forms equilibrium mixture on heating

 Intensity of reaction decreases going down group.

 See more about hydrogen halides here.

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