Neither VSEPR nor valence bond theory is able to explain all of the stable molecular structures. In addition, some molecules, such as oxygen (O2), have been shown experimentally to be paramagnetic (weakly attracted to an external magnetic field). A third model of covalent bonds can better rationalize these phenomena. Molecular orbital theory is the theory that atomic electron orbitals in covalent bonds are replaced by electron orbitals that belong to the entire molecule. A molecular orbital is a mathematical function that gives the probability of locating an electron in a localized volume of space. Similar to atomic orbitals and hybrid orbitals, molecular orbitals can hold up to two electrons with opposite spins.
Molecular orbitals are formed when atomic orbitals combine. Just like with atomic orbital hybridization, the number of molecular orbitals formed is equal to the total number of atomic orbitals combining to form them. Consider the two 1s atomic orbitals in each of the hydrogen atoms of the diatomic hydrogen (H2) molecule. Molecular orbital theory says that these two 1s orbitals combine to form two new molecular orbitals in the hydrogen molecule. One of these molecular orbitals is at a lower energy state than the 1s orbitals. This new molecular orbital is denoted by . A molecular orbital that has a high electron density between two or more atoms due to overlap of multiple atomic orbitals is called a bonding molecular orbital. A molecular orbital that induces a low electron density between two or more atoms and is called an antibonding molecular orbital. It is at a higher energy state than the atomic orbitals it is formed from. When two 1s orbitals combine, the antibonding molecular orbital is denoted by .Bonding and Antibonding Orbital Shapes
Atomic Orbitals | Atomic Orbital Shape | Bonding Orbital Name | Bonding Orbital Shape | Antibonding Orbital Name | Antibonding Orbital Shape |
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Two atomic orbitals combine to form each molecular orbital. The bonding molecular orbitals are at a lower energy state than the antibonding orbitals, and the antibonding orbitals have nodes where there are no electrons, represented by a dotted line. The 2p orbitals can combine in any of the three Cartesian coordinate planes, which results in three sets of bonding and antibonding molecular orbitals.