Group 7: The halogens

Physical properties

Property Fluorine Chlorine Bromine Iodine
Colour / state Pale yellow/gas Green/gas Brown/liquid Dark grey/solid
Colour in aqueous solution - Pale green Orange Dark brown
Electronic configuration $\left[He\right]2s^22p^6$ $\left[Ne\right]3s^23p^5$ $\left[Ar\right]3d^{10}4s^24p^5$ $\left[Kr\right]4d^{10}5s^25p^5$
Atomic radius/ionic radius Small Medium Medium Large
1st IE High Medium Medium Low
1st Electron affinity High Medium Medium Low
Electronegativity 4.0 3.0 2.8 2.5
Reactivity/oxidising power High High Medium Low
  • Colour becomes darker as van de wall forces increase, colour intensifies
  • Atomic radius increases as one goes down the group
  • IE also decreases down the group due to shielding
  • EA increases down the group as electronegativity decreases
  • Electronegativity decreases down the groups as shielding & lower nuclear charge exists

Bonding in halogen compounds

Halogens form ionic bonds with all metals, with an exception of group 3 (which form a ionic bond with fluorine, but with chlorine a covalent bond, $Al_2Cl_6$). At room temperature this exists as a dimer in liquid, & remains as a dimer at vapour. At very high temperatures forms a monomer ($AlCl_3$) This happens as aluminium (with 6 in the other shell) bonds together to make a full outer shell where temperature and excitiation permit

Fluorine always forms a -1 oxidation state (as it the most electronegative element). Other halogens can form multiple oxidation states dependent on difference in electronegativity. If chlorine reacts with oxygen, it can form $NaClO$, or $NaClO_3$, with chlorine taking a +1 or +5 respectively (as it is the lowest priority)

Oxidation states exist due to a difference in the number of things attached to the halogen

Reactions with water

  • Chlorine reacts with water to give weak acid solutions containing $HCl$ & $HClO$ reversibly
  • It is a disproportionation reaction – when reduction & oxidation occurs at the same time
  • Bromine also does the same, but iodine does not do this. fluorine forms HF & oxygen

Reactions with Alkali

At room temperature, disproportionation occurs:

$2NaOH+Cl_2\rightarrow NaCl+NaClO+H_2O$

With hot $NaOH$

$6NaOH+3Cl_2\rightarrow 5NaCl+3H_2O$

Tests for halides

Halide ion Colour of precipitate with silver nitrate Solubility in ammonia
Chlorine White precipitate Dissolves in ammonia
Bromine Pale yellow precipitate Partly dissolves
Iodine Yellow precipitate Not dissolved

Uses for halogens


  • Water purification
  • Bleach
  • Solvents
  • Polymers
  • $CFC$s

Fluorine ($F_2$)

  • $CFC$s
  • $HCFC$s (to replace the $CFC$s)
  • Polymers

Fluorine ($F^-$)

  • Helps prevent tooth decay
  • Tin fluoride in toothpaste
  • Sodium fluoride is added to water

Hydrogen fluoride ($HF$)

  • Etching glass

Silver bromide $AgBr$

  • Photographic film