AX2E2

AX2E2 is a shorthand notation used in VSEPR theory to describe the arrangement around a central atom A that is bonded to two external atoms X and has two nonbonding electron pairs E. The total number of electron domains around A is four. In this arrangement, the electron-domain geometry is tetrahedral, but the molecular geometry is bent (angular) because the two lone pairs occupy two of the four positions, compressing the X–A–X angle.

The central atom in AX2E2 is typically sp3 hybridized. Two of the sp3 orbitals accommodate the lone pairs, while the remaining two form the A–X bonds. As a result, the real-space shape of the molecule is bent rather than linear or tetrahedral.

Common examples of AX2E2 molecules are water (H2O) and hydrogen sulfide (H2S). Both have two bonded atoms and two lone pairs on the central atom. Water features a bond angle around 104.5 degrees, smaller than the ideal tetrahedral angle due to lone-pair repulsion; H2S exhibits a somewhat smaller angle, reflecting weaker lone-pair effects and differences in electronegativity.

Polarity is typical for AX2E2 species with identical X atoms, since the bent shape prevents complete cancellation of dipole moments. Consequently, molecules like H2O are strongly polar. The AX2E2 arrangement highlights how lone pairs influence geometry and properties even when only two bonds are present.