Interhalogen



The halogens react with each other to form interhalogen compounds.

The general formula of most interhalogen compounds is XYn, where n = 1, 3, 5 or 7 (X is the less anions.

Diatomic interhalogens

The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The element, X, being oxidised and having a partial positive charge. Most combinations of F, Cl, Br and I are known, but not all are stable.

  • Chlorine monofluoride (ClF), the lightest interhalogen compound, is a colorless gas with a normal boiling point of −100 °C.
  • Bromine monofluoride (BrF) has not been obtained pure — it dissociates into the trifluoride and free fluorine to form a pentafluoride.
  • Iodine monofluoride (IF) is unstable and AgF → IF + AgI at 0 °C.
  • Bromine monochloride (BrCl) is a red-brown gas with a boiling point of 5 °C.
  • crystal structure of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains.
  • Iodine monobromide (IBr) is made by direct combination of the elements to form a dark red crystalline solid. It melts at 42° C and boils at 116° C to form a partially dissociated vapour.

Tetra-atomic interhalogens

  • nickel tube. It reacts more violently than fluorine, often explosively. The molecule is planar and T-shaped.
  • organic chemistry as a fluorinating agent. It has the same molecular shape as chlorine trifluoride.
  • Iodine trifluoride (IF3) is a yellow solid which decomposes above −28 °C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5. Xe can be used. Not much is known about iodine trifluoride as it is so unstable.
  • Iodine trichloride (ICl3) forms lemon yellow crystals which can be melted under pressure to a brown liquid. It can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with many metal chlorides to form tetrachloriodides, and hydrolyses in water. The molecule is a planar dimer, with each iodine atom surrounded by four chlorine atoms.

Hexa-atomic interhalogens

  • nonmetals.
  • Iodine pentafluoride (IF5) is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with silver fluoride. It is highly reactive, even slowly with glass. It reacts with elements, oxides and carbon halides. The molecule has the form of a tetragonal pyramid.

Octa-atomic interhalogens

  • Iodine heptafluoride (IF7) is a colourless gas. It is made by reacting the pentafluoride with fluorine. IF7 is chemically inert, having no lone pair of electrons in the valency shell; in this it resembles sulfur hexafluoride. The molecule is a pentagonal bipyramid. This compound is the only interhalogen compound possible where the larger atom is carrying seven of the smaller atoms.
  • All attempts to form bromine heptafluoride have met with failure; instead, bromine pentafluoride and fluorine gas are produced.

Summary of known interhalogens

F Cl Br I
F
F2
Cl
ClF, Cl2
Br
BrF, Br2
I
IF, IF3, IF5, IF7
I2

References

  • Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. 
 
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