I2
I₂, diatomic iodine or molecular iodine, is a homonuclear diatomic molecule consisting of two iodine atoms covalently bonded together, representing the elemental form of the halogen element iodine (atomic number 53).[1] With the chemical formula I₂ and CAS number 7553-56-2, it appears as a purple-to-black crystalline solid with a metallic luster and a sharp, characteristic odor at standard temperature and pressure.[2] This solid readily sublimes upon mild heating, releasing a distinctive violet vapor, and has a molecular weight of 253.80894 g/mol.[1][2] I₂ exhibits key physical properties including a melting point of 113°C and a boiling point of 185°C, though it is more commonly observed to transition directly from solid to gas due to its high vapor pressure of 0.3 mm Hg at 25°C.[2] Chemically, it is a strong oxidizing agent that reacts with reducing agents, metals, and certain organic compounds, forming iodides and polyiodides, but it is the least reactive of the stable halogens owing to the weak I–I bond strength of approximately 151 kJ/mol.[3][4] Naturally occurring iodine is extracted primarily from seawater, brine wells, and mineral deposits, with global production centered in regions like Chile and Japan (as of 2024).[5] As a versatile compound, I₂ plays critical roles in disinfection and purification, serving as an effective antimicrobial agent in water treatment, antiseptics, and medical applications such as skin soaps and bandages due to its broad-spectrum activity against bacteria, viruses, and fungi.[6] It is also essential in industrial processes, including the manufacture of dyes, photographic films, and pharmaceuticals, as well as in analytical chemistry for starch testing where it produces a characteristic blue-black complex.[6] Additionally, while elemental I₂ itself is not directly incorporated into biological systems, iodine in ionic forms derived from it is vital for thyroid hormone synthesis, underscoring its indirect importance in human health.[6]Structure and bonding
Molecular geometry
I₂ is a homonuclear diatomic molecule exhibiting linear geometry with a bond angle of 180° due to the symmetric arrangement of its two iodine atoms connected by a single covalent bond.[7] According to valence bond theory, the I–I bond forms through the end-to-end overlap of 5p orbitals from each iodine atom, creating a σ bond, while the remaining valence electrons occupy non-bonding lone pairs on each atom. In molecular orbital theory, the bonding arises from the combination of 5s and 5p atomic orbitals, yielding a σ bonding orbital (primarily from 5p_z) that is occupied, along with filled π bonding and non-bonding orbitals, resulting in a bond order of 1; the highest occupied molecular orbitals are degenerate π* antibonding orbitals derived from 5p_x and 5p_y.[8] The equilibrium bond length in the gas phase is 266.5 pm.[7] In the solid state, this distance increases slightly to approximately 271.5 pm owing to intermolecular van der Waals interactions and crystal packing effects that elongate the intramolecular bond.[9] The characteristic purple color of gaseous and solution-phase I₂ stems from electronic transitions in the visible spectrum, particularly the promotion of an electron from the filled π* orbitals to an antibonding σ* orbital (the B ← X transition), with maximum absorption near 520 nm in the yellow-green region, transmitting purple light.[10] In its solid form, I₂ adopts an orthorhombic crystal structure (space group Cmce), consisting of layers of aligned I₂ molecules oriented nearly parallel within planes, stacked via weak van der Waals forces, which contributes to its layered cleavage and metallic luster.[11]Bond dissociation energy
The bond dissociation energy of diiodine (I₂), defined as the standard enthalpy change for the gas-phase homolytic cleavage I₂(g) → 2I(g), is 151 kJ/mol at 298 K. This value represents the lowest bond strength among the diatomic halogens, reflecting the energetic cost to break the I–I sigma bond formed by overlap of the 5p orbitals on each iodine atom.[4][7] For comparison, the bond dissociation energies of the other halogens follow a trend of decreasing strength down the group, as shown in the table below:| Molecule | Bond Dissociation Energy (kJ/mol at 298 K) |
|---|---|
| F₂ | 159 |
| Cl₂ | 243 |
| Br₂ | 193 |
| I₂ | 151 |