Problem 19 - Chemistry An Atoms First Approach

Question

Why does the molecular orbital model do a better job in explaining the bonding in \(\mathrm{NO}^{-}\) and \(\mathrm{NO}\) than the hybrid orbital model?

Step-by-Step Solution

Answer

The molecular orbital (MO) model does a better job in explaining the bonding in \(NO^{-}\) and \(NO\) than the hybrid orbital model because it accurately predicts their non-integral bond orders and the effect of unpaired electrons on bond strength. The MO model considers the combination of atomic orbitals, generating molecular orbitals categorized into bonding, antibonding, and non-bonding orbitals, which provides a more precise assessment of bond order and bond strength in these molecules. In contrast, the hybrid orbital model, with its localized bonding approach, cannot account for non-integral bond orders and fails to explain the presence or influence of unpaired electrons in \(NO\) and \(NO^{-}\).

Step 1: Brief Introduction of Molecular Orbital and Hybrid Orbital Models

The molecular orbital (MO) model and the hybrid orbital model are both theoretical models used to describe the electronic structure and bonding in molecules. The Molecular Orbital model considers the overlap and combination of atomic orbitals from individual atoms resulting in delocalized molecular orbitals that encompass the entire molecule. On the other hand, the Hybrid Orbital model focuses on the concept of hybridization, where atomic orbitals from one atom mix to form hybrid orbitals, which then overlap with atomic or hybrid orbitals from other atoms resulting in localized bonds.

Step 2: Discussing the Molecular Orbital Model for NO and \(NO^{-}\)

For the MO model, bonding in nitric oxide anionic species (\(NO^{-}\)) and neutral nitric oxide (\(NO\)) can be described by combining the atomic orbitals of nitrogen (N) and oxygen (O) atoms. The combination of atomic orbitals generates molecular orbitals, which are categorized into bonding, antibonding, and non-bonding orbitals. In the case of \(NO\), the molecular orbital diagram can be constructed by filling the molecular orbitals in the order of increasing energy, resulting in 11 electrons in bonding orbitals and 4 electrons in antibonding orbitals. This yields a bond order of 3.5, suggesting a stable and relatively strong bond between N and O. When an extra electron is added to form \(NO^{-}\), the bond order decreases to 3, still indicating a stable molecule but with a slightly weaker bond compared to neutral \(NO\).

Step 3: Analyzing the Hybrid Orbital Model for NO and \(NO^{-}\)

In the hybrid orbital model, we can analyze the bonding interactions between N and O in both \(NO\) and \(NO^{-}\) by considering the hybridization state of each atom. Nitrogen, with 5 valence electrons, will typically engage in 3sp² or 4sp³ hybridization (depending on its bonding interactions), while oxygen, with 6 valence electrons, will generally involve 2sp² or 3sp³ hybridization. However, this localized bonding approach struggles to accurately predict the bond order in \(NO\) and \(NO^{-}\), as it does not account for the non-integral bond order nature (the bond order of 3.5 and 3, respectively) observed for these molecules. Additionally, this model cannot explain the unpaired electron in \(NO\) and how an additional electron in \(NO^{-}\) affects the bond strength and order.

Step 4: Conclusion: Advantages of Molecular Orbital Model Over Hybrid Orbital Model for NO and \(NO^{-}\) Bonding

In conclusion, the molecular orbital model provides a more accurate description of the bonding in nitric oxide anionic species (\(NO^{-}\)) and neutral nitric oxide (\(NO\)) than the hybrid orbital model. This is because the molecular orbital model considers the combination of atomic orbitals, which generates bonding, antibonding, and non-bonding molecular orbitals, allowing us to predict bond order and bond strength more precisely for these molecules. Additionally, the MO model provides a better description of the unpaired electron in \(NO\) and how an additional electron in \(NO^{-}\) impacts the bond strength and order. On the other hand, the hybrid orbital model's localized bonding approach struggles to account for the non-integral bond order nature in these molecules and does not explain the presence or effect of unpaired electrons.