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A Lewis structure is a representation of a molecule or an ion showing the arrangement of electrons around atoms, including the lone pairs and bond pairs of electrons. In a Lewis structure, each atom is represented by its element symbol and is surrounded by the electron pairs (which can be shown as dots or lines).

The I−3 ion consists of three iodine (I) atoms forming a singly bonded linear structure with two extra electrons on each iodine atom (as iodine has a total of 7 electrons). In I−3 ion, the central iodine atom shares one electron each with the two adjacent iodine atoms, forming single bonds. Each iodine atom now has 2 bond pairs and 3 lone pairs (3 pairs of unshared electrons). The Lewis structure of I−3 ion can be drawn as: I − I − I Where each dash (-) represents a bond pair of electrons.

Let's now try to draw a Lewis structure for the hypothetical F−3 ion, which consists of three fluorine (F) atoms. Fluorine has a total of 7 electrons, which can form one bond by sharing one electron. For the F−3 ion to exist, it would have three fluorine atoms sharing 1 electron each to form single bonds, and the rest of the electrons would be lone pairs. But this arrangement would lead to 10 electrons around a central fluorine atom, giving each fluorine atom an expanded octet, which is not possible due to the lack of available orbitals in fluorine's 2p valence shell.

The I−3 ion is stable due to the presence of single bonds and no violation of the octet rule. Iodine atoms can extend their octet and form larger structures due to d-orbitals, which are needed for accommodating additional electrons. On the other hand, the hypothetical F−3 ion has an unstable structure, as the central fluorine atom would have to accommodate 10 electrons, violating the octet rule. In reality, such a structure does not exist because fluorine in the 2p valence shell lacks any vacant d-orbitals that are needed for an expanded octet. Thus F−3 ion does not form due to the lack of orbitals to accommodate the expanded octet. In summary, F−3 ion does not form due to a violation of the octet rule caused by the lack of available orbitals in fluorine's 2p valence shell. Whereas the I−3 ion is stable and does not violate the octet rule as its larger size allows it to accommodate an expanded octet.