Good theories, or models, of covalent bonding account for the energy states of different bonds, the energy states of electrons in a molecule, the bond angles between atoms, and the overall shape of the molecule itself. No theory is perfect, but the best theories can explain experimental observations and predict properties and behaviors of molecules. Three commonly used bonding models are the valence bond theory, the valence shell electron pair repulsion (VSEPR) theory, and the molecular orbital theory. The valence bond theory and VSEPR theory are relatively simple to use and can be used to predict molecular shape. The molecular orbital theory is more complicated but can explain physical properties that the first two theories cannot, such as paramagnetism shown by some molecules.
At A Glance
Valence bond theory is a model of covalent bonds in which atomic orbitals overlap. The bond formed when two orbitals overlap end to end is called a sigma bond (σ bond). The bond formed when two orbitals overlap side by side on the same plane is a pi bond (π bond).
- VSEPR theory predicts molecular shapes based on repulsive forces between electron pairs. According to VSEPR theory, electron pairs around a central atom maximize the distance between them.
VSEPR theory and Lewis structures can be used to predict shapes of molecules. The bonding pairs and lone pairs of a molecule determine the overall shape.
- VSEPR theory can also predict shapes due to polar covalent bonds. A dipole moment is a vector that describes the charge magnitude and direction on a polar covalent bond. When dipole moments of a molecule cancel each other, the molecule is nonpolar. If there is a net dipole moment, the molecule is polar.
Atomic orbitals combine to form hybrid atomic orbitals in a covalent bond. In an sp3 hybrid orbital, one s and three p orbitals combine.
Covalent bonds require a half-filled hybrid orbital from each atom. In an sp hybrid orbital, one s and one p orbital combine. In an sp2 hybrid orbital, one s and two p orbitals combine. In an sp3d hybrid orbital, one s, three p, and one d orbital combine. In an sp3d2 hybrid orbital, one s, three p, and two d orbitals combine.
- VSEPR theory and hybrid orbitals can explain the structure of multiple bonds. Double bonds are formed from one pi (π) bond and one sigma (σ) bond together. A triple bond is formed by one σ bond and two π bonds. A multiple bond adds rigidity to the bond.
Molecular orbitals form from bonding pairs of electrons and exist around the entire molecule instead of just one atom.
- Bond order is a counting method that gives an idea about numbers of electrons shared between atoms. A species with a higher bond order is more stable.
- Molecular bond theory can explain the properties of homonuclear diatomic molecules, such as the number of unpaired electrons and paramagnetic properties originating from the unpaired electrons.