The following is part of a molecular orbital energy-level...

The following is part of a molecular orbital energy-level diagram for MOs constructed from $1 s$ atomic orbitals. (a) What labels do we use for the two MOs shown? (b) For which of the following molecules or ions could this be the energy-level diagram: $\mathrm{H}_{2}, \mathrm{He}_{2}, \mathrm{H}_{2}{ }^{+}, \mathrm{He}_{2}{ }^{+}$, or $\mathrm{H}_{2}{ }^{-}$? (c) What is the bond order of the molecule or ion? (d) If an electron is added to the system, into which of the MOs will it be added? [Section 9.7]

The following is part of a molecular orbital energy-level diagram for MOs constructed from $1 s$ atomic orbitals.
(a) What labels do we use for the two MOs shown? (b) For which of the following molecules or ions could this be the energy-level diagram:
$\mathrm{H}_{2}, \mathrm{He}_{2}, \mathrm{H}_{2}{ }^{+}, \mathrm{He}_{2}{ }^{+}$, or $\mathrm{H}_{2}{ }^{-}$?
(c) What is the bond order of the molecule or ion? (d) If an electron is added to the system, into which of the MOs will it be added? [Section 9.7]

Key Concepts

Molecular Orbital Theory

Molecular orbital theory explains how atomic orbitals combine to form molecular orbitals that extend over an entire molecule. Instead of being confined to individual atoms, electrons are considered to be delocalized over the entire molecule, and these molecular orbitals can be either lower in energy (bonding orbitals) or higher in energy (antibonding orbitals) compared to the individual atomic orbitals.

Bonding and Antibonding Orbitals

When atomic orbitals combine, they form two types of molecular orbitals: bonding orbitals (denoted often as ?, which result from in-phase combination and have electron density between the nuclei) and antibonding orbitals (denoted often as ?* or ?*, which result from out-of-phase combination and have a node between the nuclei). The bonding orbitals are energetically favorable and stabilize the molecule, while the antibonding orbitals destabilize it.

Bond Order

Bond order is a quantitative measure of the strength of a chemical bond within the molecular orbital framework and is calculated as one half the difference between the number of electrons in bonding and antibonding orbitals. A higher bond order usually indicates a stronger, more stable bond, while a bond order of zero suggests the molecule is unstable or non-existent.

Electron Configuration in Molecular Orbitals

The electron configuration in molecular orbitals involves filling the molecular orbitals in order of increasing energy, following principles analogous to those in atomic orbital filling. When electrons are added to or removed from a system (such as in ions), they are placed or removed from the highest occupied molecular orbital, which determines the properties and reactivity of the molecule or ion.

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