The molecular geometry of phosphine (PH₃) is trigonal pyramidal. This shape arises from the arrangement of its atoms and lone pairs around the central phosphorus (P) atom. Here’s a detailed breakdown of why PH₃ adopts this geometry:
1. Lewis Structure of PH₃
- Central Atom: Phosphorus (P)
- Bonding Pairs: Three P-H bonds
- Lone Pairs: One lone pair on the phosphorus atom
The Lewis structure of PH₣ can be visualized as follows:
H
/
P
|
H
|
H
With a lone pair occupying the fourth position around the phosphorus atom.
2. Electron Pair Geometry
To determine the molecular geometry, we first consider the electron pair geometry, which includes both bonding pairs and lone pairs.
- Phosphorus has four electron regions (three bonding pairs and one lone pair).
- According to VSEPR theory (Valence Shell Electron Pair Repulsion), four electron regions adopt a tetrahedral arrangement to minimize electron pair repulsion.
3. Molecular Geometry
While the electron pair geometry is tetrahedral, the molecular geometry focuses only on the positions of the atoms, not the lone pairs.
- The three hydrogen atoms form the base of a pyramid, with the phosphorus atom at the apex.
- The lone pair on phosphorus pushes the bonding pairs closer together, distorting the perfect tetrahedral shape and resulting in a trigonal pyramidal geometry.
4. Bond Angles
The presence of the lone pair affects the bond angles in PH₃.
- In a perfect tetrahedron, the bond angle would be 109.5°.
- However, the lone pair exerts greater repulsive force than bonding pairs, compressing the H-P-H bond angles to approximately 93.5°.
5. Comparison with Other Molecules
PH₃ is analogous to NH₃ (ammonia), which also has a trigonal pyramidal geometry due to a central atom with three bonding pairs and one lone pair. However, the bond angle in PH₃ is slightly smaller than in NH₃ (107.5° for NH₃) due to the larger size of the phosphorus atom.
6. Key Takeaways
FAQ Section
Why is the bond angle in PH₃ less than 109.5°?
+The lone pair on phosphorus exerts greater repulsive force than bonding pairs, compressing the H-P-H bond angles from the ideal tetrahedral angle of 109.5° to approximately 93.5°.
How does PH₃ differ from NH₃ in terms of geometry?
+Both PH₃ and NH₃ have a trigonal pyramidal geometry, but the bond angle in PH₃ (~93.5°) is smaller than in NH₃ (~107.5°) due to the larger size of the phosphorus atom compared to nitrogen.
What role does the lone pair play in PH₃'s geometry?
+The lone pair on phosphorus occupies space and repels the bonding pairs more strongly than they repel each other, resulting in a trigonal pyramidal shape with compressed bond angles.
Can PH₃ be classified as a polar molecule?
+Yes, PH₃ is polar. The trigonal pyramidal geometry results in an uneven distribution of charge, with a net dipole moment due to the lone pair and the P-H bonds.
In summary, the molecular geometry of PH₃ is trigonal pyramidal, with a bond angle of approximately 93.5°, determined by the arrangement of three bonding pairs and one lone pair around the phosphorus atom. This shape is a direct consequence of VSEPR theory and the repulsive effects of the lone pair.
