Accepted answer
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Geometry and bond angles
What is the bond angle of ax4e2? The AX4E2 arrangement corresponds to a square-planar molecular shape, so adjacent X–A–X bond angles are 90° and opposite X–A–X bond angles are 180° (idealized VSEPR).
Domain count and shape assignment
\[
\text{Steric number (electron domains)} = X + E
\]
\[
\text{AX}_4\text{E}_2 \Rightarrow X+E=4+2=6
\]
Six electron domains correspond to an octahedral electron-domain geometry. With two lone pairs (E2), the observed molecular geometry becomes square planar.
Electron-domain picture
- Electron-domain geometry: octahedral (six regions of electron density around A).
- Lone-pair placement: two lone pairs occupy opposite (trans) positions to maximize separation.
- Molecular geometry: four bonded atoms (X4) remain in one plane, forming a square around A.
Bond-angle values for AX4E2
| Angle type | Definition | Ideal value |
|---|---|---|
| Adjacent X–A–X | Angle between neighboring ligands in the square plane | 90° |
| Opposite X–A–X | Angle between ligands across the square (trans in the plane) | 180° |
Representative example
Xenon tetrafluoride, XeF4, is a common main-group example with AX4E2 counting: four Xe–F bonds and two lone pairs on xenon. The lone pairs occupy axial positions (above and below the plane), leaving the four fluorine atoms in a square plane with 90° and 180° F–Xe–F angles.
Common pitfalls
- Electron-domain geometry versus molecular geometry: AX4E2 has octahedral electron domains but square-planar molecular shape.
- Lone-pair placement: the lowest-repulsion arrangement places the two lone pairs opposite each other, not adjacent.
- Angle expectations: tetrahedral 109.5° does not apply because the domain count is six rather than four.
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