What is the “AXE” description of the bromite (BrO 2)anion?
The Correct Answer and Explanation is :
AXE Description of the Bromite ((\text{BrO}_2^-)) Anion
The AXE notation is a system used in VSEPR (Valence Shell Electron Pair Repulsion) theory to describe molecular geometry. In this notation:
- A represents the central atom.
- X represents the number of bonded atoms.
- E represents the number of lone pairs on the central atom.
For the bromite anion ((\text{BrO}_2^-)):
- Identify the central atom:
- The bromine (Br) atom is the central atom.
- Determine the number of bonded atoms (X):
- Bromine is bonded to two oxygen atoms.
- Determine the number of lone pairs on the central atom (E):
- Bromine belongs to Group 17 (halogens), meaning it has 7 valence electrons.
- The two oxygen atoms each form a single bond (with one having a negative charge), utilizing 2 valence electrons from Br.
- The remaining 5 valence electrons on Br form two lone pairs and one additional electron is used for bonding.
- Thus, Br has two lone pairs.
- Write the AXE notation:
- A = 1 (Bromine)
- X = 2 (Two oxygen atoms bonded)
- E = 2 (Two lone pairs)
- AXE notation: AX₂E₂
Explanation of the Geometry:
- According to VSEPR theory, an AX₂E₂ molecule adopts a bent or V-shaped geometry.
- The lone pairs repel the bonded oxygen atoms, forcing them into a bent configuration.
- The ideal bond angle for an AX₂E₂ system is around 104-120°, similar to water ((\text{H}_2\text{O})).
- The presence of lone pairs reduces the bond angle slightly due to increased electron repulsion.
Conclusion:
The AXE notation of the bromite anion ((\text{BrO}_2^-)) is AX₂E₂, leading to a bent molecular shape due to lone pair repulsions.