Arsenic compounds: arsenic triiodide

arsenic symbol icon

The arsenic in arsenic triiodide formally is in the oxidation state 3.

Crystal structure of arsenic triiodide

Arsenic triiodide

  • Formula as often written: AsI3
  • Hill system formula: As1I3
  • CAS registry number: [7784-45-4]
  • Formula weight: 455.635
  • Class: iodide


  • arsenic triiodide
  • arsenic(III) iodide
  • arsenic iodide

Physical properties

  • Colour: red
  • Appearance: crystalline solid
  • Melting point: 141°C
  • Boiling point: 400°C; 424°C
  • Density: 4730 kg m-3; 4390 kg m-3


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Element analysis

Element percentages for the elements in arsenic triiodide
Element %
As 16.44
I 83.56


AsCl3 + 3KI → AsCl3 + 3KCl

Arsenic(III) triiodide can be made by the direct reaction of the elements, but is also conveniently made by the reaction between arsenic(III) chloride and iodide salts. The arsenic(III) chloride is formed simply enough by dissolving the oxide As2O3 in concentrated hydrochloric acid. This is mixed with a solution of potassium iodide in water. The yield is about 90%. Arsenic(III) triiodide is orange-red and melts at 149°C. Its boiling point is 400°C and it is slightly water-soluble.

Solid state structure

  • Geometry of arsenic:
  • Prototypical structure:

Crystal structure of arsenic triiodide

Isotope pattern

What follows is the calculated isotope pattern for the AsI3 unit with the most intense ion set to 100%.

Formula: As1I3

mass  %
456 100.0 __________________________________________________


The data on these compounds pages are assembled and adapted from the primary literature and several other sources including the following.

  • R.T. Sanderson in Chemical Periodicity, Reinhold, New York, USA, 1960.
  • N.N. Greenwood and A. Earnshaw in Chemistry of the Elements, 2nd edition, Butterworth, UK, 1997.
  • F.A. Cotton, G. Wilkinson, C.A. Murillo, and M. Bochmann, in Advanced Inorganic Chemistry, John Wiley & Sons, 1999.
  • A.F. Trotman-Dickenson, (ed.) in Comprehensive Inorganic Chemistry, Pergamon, Oxford, UK, 1973.
  • R.W.G. Wyckoff, in Crystal Structures, volume 1, Interscience, John Wiley & Sons, 1963.
  • A.R.West in Basic solid state chemistry Chemistry, John Wiley & Sons, 1999.
  • A.F. Wells in Structural inorganic chemistry, 4th edition, Oxford, UK, 1975.
  • J.D.H. Donnay, (ed.) in Crystal data determinative tables, ACA monograph number 5, American Crystallographic Association, USA, 1963.
  • D.R. Lide, (ed.) in Chemical Rubber Company handbook of chemistry and physics, CRC Press, Boca Raton, Florida, USA, 77th edition, 1996.
  • J.W. Mellor in A comprehensive treatise on inorganic and theoretical chemistry, volumes 1-16, Longmans, London, UK, 1922-1937.
  • J.E. Macintyre (ed.) in Dictionary of inorganic compounds, volumes 1-3, Chapman & Hall, London, UK, 1992.

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arsenic atomic number