Methane, CH4, has the ideal tetrahedral angle (109.5°) and is a nonpolar molecule, while NH3 and H2O show a deviation from the ideal bond angle. Explain the cause and why water has a greater deviation than ammonia.Methane, CH4, has the ideal tetrahedral angle (109.5°) and is a nonpolar molecule, while NH3 and H2O show a deviation from the ideal bond angle. Explain the cause and why water has a greater deviation than ammonia.
The Correct Answer and Explanation is:
Correct Answer:
The deviation from the ideal tetrahedral bond angle in NH₃ and H₂O is due to the presence of lone pairs of electrons on the central atom. Lone pairs occupy more space than bonding pairs, causing greater repulsion and compressing the bond angles between atoms. Water (H₂O) has two lone pairs, while ammonia (NH₃) has only one, leading to a greater deviation in water.
Detailed Explanation (300+ words):
Methane (CH₄) has a central carbon atom bonded to four hydrogen atoms with no lone pairs on the carbon. According to Valence Shell Electron Pair Repulsion (VSEPR) theory, electron pairs (bonding and lone pairs) around a central atom repel each other and will arrange themselves as far apart as possible. In CH₄, all four electron domains are bonding pairs, which adopt a tetrahedral geometry with ideal bond angles of 109.5°. Since the molecule is symmetrical and all outer atoms are the same, CH₄ is nonpolar.
Ammonia (NH₃), in contrast, has a nitrogen atom with three bonding pairs and one lone pair. This gives it a total of four electron domains, like CH₄, and it retains a tetrahedral electron geometry. However, because a lone pair occupies more space than a bonding pair (due to its higher electron density and absence of nuclear attraction from a second nucleus), it pushes the bonding pairs closer together. This causes the H–N–H bond angle in ammonia to decrease from 109.5° to about 107°.
Water (H₂O) shows an even greater deviation. The oxygen atom in H₂O has two bonding pairs and two lone pairs, still a total of four electron domains and thus a tetrahedral electron geometry. However, the two lone pairs exert even more repulsion on the bonding pairs than the single lone pair in NH₃. As a result, the H–O–H bond angle in water is reduced further to about 104.5°.
In summary, the bond angle deviation increases with the number of lone pairs due to increased lone pair–bond pair repulsion. Therefore, water has a greater deviation from the ideal tetrahedral angle than ammonia because it has two lone pairs instead of one.