Place the following molecules or ions in order of decreasing bond order: (a) \(\mathrm{NO}\) bond in \(\mathrm{NO}, \mathrm{NO}_{2}, \mathrm{NO}_{3}^{-}\); (b) \(\mathrm{CC}\) bond in \(\mathrm{C}_{2} \mathrm{H}_{2}, \mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{C}_{2} \mathrm{H}_{6} ;\) (c) \(\mathrm{CO}\) bond in \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{2} \mathrm{O}, \mathrm{CH}_{3} \mathrm{OCH}_{3}\). Explain your reasoning.

Understanding the difference between bonding and anti-bonding electrons is crucial in determining the stability and strength of a chemical bond. Bonding electrons are those that occupy the molecular orbitals closer to the nuclei of the bonded atoms and contribute to the bond formation. These electrons lower the energy between atoms, thus establishing a stable connection.

Conversely, anti-bonding electrons are found in molecular orbitals that are higher in energy and are located further away from the nuclei. They actually work against bond formation by increasing the energy of the system, leading to destabilization.

The bond order, a pivotal concept in molecular chemistry, can be understood as the difference between the number of bonding electrons and anti-bonding electrons, all divided by two. A higher bond order corresponds to a stronger, more stable bond. For instance, when comparing molecules with single, double, and triple bonds, the bond order would be 1, 2, and 3, respectively, reflecting ascending strength and stability.

Molecular Orbital Theory provides a nuanced understanding of electron distribution within a molecule, predicting chemical properties such as bond order, magnetism, and bonding nature. In this framework, atomic orbitals from each atom combine to form molecular orbitals, which can be of two types: bonding and anti-bonding, as previously mentioned.

According to this theory, when two atoms bond, their atomic orbitals overlap to create a molecular orbital that can be occupied by two electrons with opposite spins. If this molecular orbital is of lower energy, it is considered a bonding orbital. If it is of higher energy, it is an anti-bonding orbital. The electrons in the bonding orbitals stabilize the molecule, whereas electrons in anti-bonding orbitals have the opposite effect.

Bonding Order in Molecular Orbital Theory

Within Molecular Orbital Theory, bond order becomes a quantitative measure of the molecule's stability, calculated by subtracting the number of anti-bonding electrons from the number of bonding electrons and then dividing by two.

Resonance structures come into play when a single Lewis structure is insufficient to depict the true electron distribution within a molecule. This phenomenon is particularly relevant in systems with conjugated pi electrons or delocalized charge.

Resonance structures are multiple valid Lewis structures that collectively represent a more accurate picture of the electron distribution. These structures are not real intermediates or isomers of the molecule; instead, the actual molecule is a hybrid of all the resonance structures, with electron density distributed over the molecule in a way that stabilizes it.

Resonance and Bond Order

The concept of resonance can affect bond order. For example, in a molecule like nitrate (o3⁻), resonance leads to a situation where all N-O bonds have the same length and energy, indicative of a bond order between that of a single and a double bond. This delocalization over several structures can lower the energy of the system, making the molecule more stable than it would be if the electrons were localized as depicted in a single Lewis structure.