The Lewis diagram for AsO2- is: :8~4s=8 Recall that for predicting geometry, double and triple bonds electron pair region count as only one. The electron-pair geometry around the As atom in AsO2- is trigonal planar. There is an unshared pair around the central atom, so the geometry of the AsO2- molecule is bent.
The Correct Answer and Explanation is:
The correct answer is 1.
Explanation
The question asks for the number of unshared electron pairs, also known as lone pairs, on the central arsenic (As) atom in the arsenite ion (AsO₂⁻). To determine this, we can use the information provided about the molecule’s geometry, which is based on Valence Shell Electron Pair Repulsion (VSEPR) theory.
First, the problem states that the electron-pair geometry around the arsenic atom is trigonal planar. An electron-pair geometry describes the arrangement of all electron domains (both bonding pairs and lone pairs) around the central atom. A trigonal planar arrangement always corresponds to a total of three electron domains.
Next, we identify how many of these domains are bonding pairs. The arsenic atom is bonded to two oxygen atoms. According to VSEPR theory, each bonded atom counts as one electron domain, regardless of whether the bond is single, double, or triple. Since there are two oxygen atoms attached to the central arsenic, there are two bonding domains.
To find the number of unshared or lone pairs, we subtract the number of bonding domains from the total number of electron domains:
Total Electron Domains = 3 (from the trigonal planar geometry)
Bonding Domains = 2 (from the two oxygen atoms)
Unshared Pairs = Total Domains – Bonding Domains = 3 – 2 = 1
Therefore, there must be one unshared pair of electrons on the central arsenic atom.
This conclusion is further supported by the stated molecular geometry, which is bent. For a molecule with three electron domains, if one of them is a lone pair (an AX₂E₁ configuration), the lone pair repels the bonding pairs, forcing the atoms into a bent or V-shaped arrangement. If there were no lone pairs, the molecular geometry would be the same as the electron geometry, which would be trigonal planar, not bent. The provided Lewis diagram is misleading as it omits this essential lone pair on the arsenic atom.
