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Chapter 7: Problem 74

Consider the following molecules: \(\mathrm{SiH}_{4}, \mathrm{PH}_{3}, \mathrm{H}_{2} \mathrm{~S}\). In each case, a central atom is surrounded by four electron pairs. In which of these molecules would you expect the bond angle to be less than \(109.5^{\circ}\) ? Explain your reasoning.

Short Answer

Expert verified
The molecules with bond angles less than \(109.5^{\circ}\) are \(\mathrm{PH}_{3}\) and \(\mathrm{H}_{2}\mathrm{S}\).

Step by step solution

01

Determine the molecular geometry of each molecule

We first need to identify the molecular geometry of each molecule. We can do this using VSEPR (Valence Shell Electron Pair Repulsion) theory, which focuses on the repulsion between electron pairs around a central atom. We can count the number of lone pairs and bond pairs for each central atom: For \(\mathrm{SiH}_{4}\): - Si is the central atom - 4 bond pairs with H atoms - 0 lone pairs For \(\mathrm{PH}_{3}\): - P is the central atom - 3 bond pairs with H atoms - 1 lone pair For \(\mathrm{H}_{2}\mathrm{S}\): - S is the central atom - 2 bond pairs with H atoms - 2 lone pairs
02

Analyze the effect of lone pairs and bond pairs on bond angles

According to VSEPR theory, lone pairs of electrons occupy more space around the central atom than bonding pairs and repel more strongly. Therefore, lone pairs tend to compress bond angles, making them smaller. In \(\mathrm{SiH}_{4}\), there are no lone pairs, so there is no compression of bond angles. The bond angles in a tetrahedral geometry are equal to \(109.5^{\circ}\). In both \(\mathrm{PH}_{3}\) and \(\mathrm{H}_{2}\mathrm{S}\), there are lone pairs present. This presence results in a compression of bond angles.
03

Determine the molecule with bond angles less than \(109.5^{\circ}\)

Based on the presence of lone pairs in \(\mathrm{PH}_{3}\) and \(\mathrm{H}_{2}\mathrm{S}\), we can expect their bond angles to be less than \(109.5^{\circ}\). In contrast, \(\mathrm{SiH}_{4}\) has no lone pairs, so its bond angles will be closer to the ideal tetrahedral angle of \(109.5^{\circ}\). So the molecules with bond angles less than \(109.5^{\circ}\) are \(\mathrm{PH}_{3}\) and \(\mathrm{H}_{2}\mathrm{S}\).

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