NH3, PH3, et AsH3 sont isolélectroniques (même formule de...

$\mathrm{NH}_3$, $\mathrm{PH}_3$, et $\mathrm{AsH}_3$ sont isolélectroniques (même formule de Lewis). Quelle est leur géométrie commune? Les angles formés par leurs liaisons sont : $\mathrm{NH}_3: 107^{\circ} 30^{\prime} \mathrm{PH}_3: 93^{\circ} 30^{\prime} \mathrm{AsH}_3: 91^{\circ} 30^{\prime}$ $>$ Quelles valeurs attend-on a priori ? > Comment peut-on expliquer ces différences entre prévision et réalité, ainsi qu'entre les trois molécules?

$\mathrm{NH}_3$, $\mathrm{PH}_3$, et $\mathrm{AsH}_3$ sont isolélectroniques (même formule de Lewis). Quelle est leur géométrie commune?
Les angles formés par leurs liaisons sont :
$\mathrm{NH}_3: 107^{\circ} 30^{\prime} \mathrm{PH}_3: 93^{\circ} 30^{\prime} \mathrm{AsH}_3: 91^{\circ} 30^{\prime}$
$>$ Quelles valeurs attend-on a priori ?
> Comment peut-on expliquer ces différences entre prévision et réalité, ainsi qu'entre les trois molécules?

Key Concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the three-dimensional shape of a molecule based on electron pair repulsions. In molecules with a central atom surrounded by bonding pairs and lone pairs, the lone pairs repel the bonded pairs more strongly, leading to deviations from the idealized geometries. The theory provides an initial expectation for bond angles and shapes before considering other effects such as orbital hybridization.

Lone Pair Repulsion

Lone pairs of electrons occupy more space than bonding pairs, and their repulsion compresses the bond angles between adjacent bonding pairs. In the case of molecules like NH3, PH3, and AsH3, the lone pair present on the central atom pushes the bonding hydrogen atoms closer together, reducing the bond angles from the ideal tetrahedral value that would be expected if only bonding pairs were present.

Hybridization and Orbital Considerations

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals that dictate molecular geometry. In lighter atoms like nitrogen, sp3 hybridization leads to a geometry with bond angles slightly less than the ideal tetrahedral angle due to lone pair repulsion. Conversely, heavier atoms like phosphorus and arsenic show less effective hybridization, and their bonding involves orbitals that are more p-character, resulting in bond angles that approach 90°.

Atomic Size and Electronegativity

The differences in atomic size and electronegativity among the central atoms also affect the molecular geometries. Smaller, more electronegative atoms such as nitrogen tend to form stronger bonds and more directional hybrid orbitals, leading to bond angles that deviate less drastically from the idealized geometry. In contrast, larger atoms like phosphorus and arsenic have diffuse orbitals, which contribute to weaker and less directional bonding, thereby producing bond angles closer to 90°.

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