Crystal Structure of Iodine

Iodine is one of two halogens that exist as a solid at room temperature. Iodine readily forms orthorhombic crystals, meaning that the three axes of the "unit cell" (a, b, and c, of the crystal) are all at right angles to each other and all of different length.

History

In 1928, the crystal structure of iodine was studied by means of X-ray diffraction. In 1966, Acta Crystallographica received report of yet further refinement based on data from a study conducted at 110 degrees Kelvin.

In the Gaseous State

In the gaseous state, Iodine is diatomic, i.e., its formula is I₂, drawn as I --- I, with the dash representing a covalent bond. The gaseous iodine bond length is 2.662 ... (1 ...ngström = 1 x 10⁻⁸ centimeters).

In the Solid State

For solid iodine, the length of the I---I bond is 2.715 ..., an elongation of nearly 2 percent. Surrounding Iodine atoms exert forces on each pair, and may be visualized as a tug-of-war, I ······ I --- I ······ I, resulting in the lengthening of the covalent bond.

Covalent Crystal

In 2004, a study in "Semiconductors and Dielectrics" explained that solid iodine is not merely a molecular crystal, but is a covalent crystal.

Dissociation Energy

Molecular crystals are held together by relatively weak van der Waals forces; dissociation energy for crystalline iodine is at least an order of magnitude greater than that.

References

• "Some Optical Properties of Iodine," (1903), by William Weber Coblentz.
• IUCr Crystallography Journals Online; Acta Crystallographica; "Refinement of the crystal structure..."
• WebElements; "Iodine - Crystal Structure"

Resources

• EnvironmentalChemistry.com; "Crystal Structure"
• Naval Research Laboratory; "Crystal Lattice Structures" - Iodine from several perspectives
• Semiconductors and Dielectrics (2004); "Iodine is a Covalent Crystal"