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The following exergonic equilibrium gives rise to the triiodide ion: . I 2 + I − ⇌ I − 3. In this reaction, iodide is viewed as a Lewis base, and the iodine is a Lewis acid.The process is analogous to the reaction of S 8 with sodium sulfide (which forms polysulfides) except that the higher polyiodides have branched structures.
The aqueous solution is highly acidic, pH of 0.1N solution is 1.1. It decomposes to arsenic trioxide, elemental arsenic and iodine when heated in air at 200 °C. The decomposition, however, commences at 100 °C and occurs with the liberation of iodine.
Expressing resonance when drawing Lewis structures may be done either by drawing each of the possible resonance forms and placing double-headed arrows between them or by using dashed lines to represent the partial bonds (although the latter is a good representation of the resonance hybrid which is not, formally speaking, a Lewis structure).
An example of a dative covalent bond is provided by the interaction between a molecule of ammonia, a Lewis base with a lone pair of electrons on the nitrogen atom, and boron trifluoride, a Lewis acid by virtue of the boron atom having an incomplete octet of electrons. In forming the adduct, the boron atom attains an octet configuration.
For example, NH 3 is a Lewis base, because it can donate its lone pair of electrons. Trimethylborane [(CH 3) 3 B] is a Lewis acid as it is capable of accepting a lone pair. In a Lewis adduct, the Lewis acid and base share an electron pair furnished by the Lewis base, forming a dative bond. [1]
The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms. [1] [2] The most common member is the triiodide ion, I −3.Other known larger polyiodides include [I 4] 2−, [I 5] −, [I 6] 2−, [I 7] −, [I 8] 2−, [I 9] −, [I 10] 2−, [I 10] 4−, [I 11] 3−, [I 12] 2−, [I 13] 3−, [I 14] 4-, [I 16] 2−, [I 22] 4−, [I 26] 3−, [I 26] 4−, [I 28] 4− and ...
When the solutions are mixed, the second reaction causes the iodine to be consumed much faster than it is generated, and only a small amount of iodine is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the iodine – starch complex appears.
Figure 2: A donor-acceptor interaction diagram illustrating construction of the triiodide anion σ natural bond orbitals from I 2 and I − fragments. In the natural bond orbital viewpoint of 3c–4e bonding, the triiodide anion is constructed from the combination of the diiodine (I 2 ) σ molecular orbitals and an iodide (I − ) lone pair.