Write the Lewis structure, identify and name the molecular shape and draw the 3D structures of the following molecules: a.) PH3 b.) H2S c.) Which molecule has the largest angle(s) between bonds? Why?
The correct answer and explanation is:
Sure! Here’s a complete answer for PH₃ and H₂S, including Lewis structures, molecular geometry, 3D shape, bond angles, and analysis:
a.) PH₃ (Phosphine)
- Lewis Structure:
H | H - P - H :- Phosphorus (P) has 5 valence electrons.
- Forms 3 single bonds with hydrogen atoms.
- One lone pair remains on phosphorus.
- Molecular Shape (VSEPR): Trigonal pyramidal
- 3D Structure:
The molecule looks like a pyramid with a lone pair at the top pushing the three H atoms downward. - Bond Angle: ~93.5°
b.) H₂S (Hydrogen sulfide)
- Lewis Structure:
H - S - H .. ..- Sulfur (S) has 6 valence electrons.
- Forms 2 single bonds with hydrogen.
- Has 2 lone pairs on sulfur.
- Molecular Shape (VSEPR): Bent (or V-shaped)
- 3D Structure:
The two H atoms are bent downward with two lone pairs on top, similar to a water molecule. - Bond Angle: ~92.1°
c.) Which molecule has the largest bond angle? Why?
- Answer: PH₃ has the larger bond angle (~93.5°) compared to H₂S (~92.1°).
- Explanation (300 words):
Molecular shape and bond angles in molecules are determined using the VSEPR (Valence Shell Electron Pair Repulsion) theory. This theory assumes that electron pairs (bonding and lone pairs) around a central atom will arrange themselves as far apart as possible to minimize repulsion.
In PH₃, the phosphorus atom has three bonded hydrogen atoms and one lone pair. The resulting shape is trigonal pyramidal. The lone pair-bond pair repulsion slightly compresses the ideal tetrahedral angle (109.5°) to about 93.5°, but because phosphorus is a larger atom with more diffuse orbitals, the repulsions are less intense.
In contrast, H₂S has two bonding pairs and two lone pairs on sulfur, leading to a bent geometry. The two lone pairs exert strong repulsive forces on the bonding pairs, causing a smaller bond angle (~92.1°). Also, sulfur is more electronegative and has smaller orbitals compared to phosphorus, resulting in more localized electron repulsion and tighter bond angles.
Thus, although both molecules are non-linear due to lone pairs, PH₃ has the larger bond angle because it has fewer lone pairs and a central atom with more diffused orbitals, which reduces electron-electron repulsion and allows for slightly wider spacing between bonds compared to H₂S.