AX2E3

AX2E3 denotes a molecular arrangement where a central atom A is bonded to two X atoms (X2) and contains three lone pairs (E) in its valence shell. In VSEPR terms, this corresponds to five electron domains around A. The electron-domain geometry is trigonal bipyramidal, but the three lone pairs prefer equatorial positions, leaving the two X substituents in the axial sites. As a result, the molecular geometry is linear, with an X–A–X bond angle of approximately 180 degrees. The central atom typically uses sp3d hybridization to accommodate five regions of electron density.

A common example is the triiodide ion, I3−, where iodine serves as both the central atom A and one of the X atoms. Other polyhalide ions with AX2E3 arrangements can occur, depending on the element and environment. If the two X atoms are identical, the molecule is symmetric and often nonpolar due to cancellation of dipole moments.

Limitations: VSEPR provides a best-fit model based on electron-pair repulsion and does not capture all subtleties of bonding, resonance, or hypervalency in all cases. The AX2E3 designation serves as a shorthand for this specific electron and molecular geometry within the broader AXnE categorization.