Answer
Molecular Orbital Diagram of HCl
Hydrogen chloride (HCl) is a heteronuclear diatomic molecule formed by the bonding of a hydrogen atom (1s orbital) and a chlorine atom (3p orbital). Since the atomic orbitals of H and Cl are significantly different in energy, only certain orbitals effectively combine to form molecular orbitals (MOs).
Atomic Orbital Composition
- Hydrogen: 1s orbital
- Chlorine: 3s, 3px, 3py, 3pz
- Only the Cl 3pz orbital overlaps significantly with H 1s to form a bonding MO.
Formation of Molecular Orbitals
- σ (sigma) bonding orbital: Formed by the overlap of H 1s and Cl 3pz.
- σ* (sigma antibonding orbital): Higher in energy, contains a node between nuclei.
- Nonbonding orbitals: Cl’s 3s, 3px, and 3py remain essentially nonbonding due to lack of suitable overlap.
Relative Energy Levels
The H 1s orbital is significantly higher in energy than the Cl 3p orbital. As a result, the bonding MO (σ) is closer in energy to Cl’s orbitals, showing that the bonding electron density is skewed toward the chlorine atom.
This confirms that HCl has a single covalent bond.
MO Density Distributions
The σ bonding MO shows a concentrated electron density between the H and Cl nuclei but with an asymmetrical shape skewed toward the more electronegative Cl atom. The σ* antibonding MO exhibits a node between the atoms with electron density outside the internuclear region.
MO Energy Diagram Overview
- Left column (H): 1s
- Right column (Cl): 3s (nonbonding), 3px, 3py (nonbonding), 3pz → interacts with H 1s
- Center (MOs):
- σ (bonding) ← from H 1s + Cl 3pz
- σ* (antibonding) ← higher energy, contains node
- Nonbonding: 3s, 3px, 3py
Summary
The molecular orbital structure of HCl demonstrates an unequal sharing of electrons due to different electronegativities and orbital energies. The bonding MO is σ-type, composed of H 1s and Cl 3pz, and the molecule includes several nonbonding orbitals from Cl that do not significantly interact with H’s orbital.