Tetrahedral molecular geometry
| Tetrahedral molecular geometry | |
|---|---|
 |
|
| Examples |
CH4, PO3− 4, SO2− 4 |
| Point group | Td |
| Steric number | 4 |
| Coordination number | 4 |
| Bond angle(s) | ≈109.5° |
In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1(−1/3) = 109.4712206...° ≈ 109.5° when all four substituents are the same, as in methane (CH4) as well as its heavier analogues. The perfectly symmetrical tetrahedron belongs to point group Td, but most tetrahedral molecules have lower symmetry. Tetrahedral molecules can be chiral.
Aside from virtually all saturated organic compounds, most compounds of Si, Ge, and Sn are tetrahedral. Often tetrahedral molecules feature multiple bonding to the outer ligands, as in xenon tetroxide (XeO4), the perchlorate ion (ClO−
4), the sulfate ion (SO2−
4), the phosphate ion (PO3−
4). Thiazyl trifluoride (SNF3) is tetrahedral, featuring a sulfur-to-nitrogen triple bond.
Other molecules have a tetrahedral arrangement of electron pairs around a central atom; for example ammonia (NH3) with the nitrogen atom surrounded by three hydrogens and one lone pair. However the usual classification considers only the bonded atoms and not the lone pair, so that ammonia is actually considered as pyramidal. The H–N–H angles are 107°, contracted from 109.5. This difference is attributed to the influence of the lone pair which exerts a greater repulsive influence than a bonded atom.
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