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a mathematical function describing the wave-like behavior of one or a pair of electrons; used to calculate the probability of finding any electron in any specific region around the atom's nucleus; it may also refer to the physical region where the electron may be
VB theory dictates that overlapping atomic orbitals of participating atoms form a chemical bond. Because of the overlap, it is probable that electrons are found in the bond (where the orbitals overlap) region.
Sigma and Pi Bonds
There are two types of overlapping orbitals: sigma and pi. Sigma bonds occur when the orbitals of two shared electrons overlap between the nuclei of two atoms; Pi bonds occur when the two overlapping orbitals are outside of the space between the nuclei (above, below, in front, and in back).
VB theory complements molecular orbital (MO) theory, which does not adhere to the VB concept that electron pairs are localized between two specific atoms in a molecule. MO theory suggests that electrons are distributed in sets of molecular orbitals that can extend over the entire molecule. MO theory can predict magnetic and ionization properties in a straightforward manner. VB theory produces similar results, but is more complicated.
An important aspect of the VB theory is the condition of maximum overlap which leads to the formation of the strongest possible bonds. This theory is used to explain the covalent bond formation in many molecules. In the F2 molecule, the F–F bond is formed by the overlap of pz orbitals of the two F atoms, each containing an unpaired electron. Since the nature of the overlapping orbitals is different in H2 and F2 molecules, bond strength and bond lengths differ between H2 and F2 molecules.
In an HF molecule, the covalent bond forms from the overlap of the 1s orbital of H and the 2pz orbital of F, each containing an unpaired electron. Mutual sharing of electrons between H and F results in a covalent bond in HF.
Covalent bond between hydrogen atoms
Each hydrogen atom has one electron. To complete their valence shells, they bond and share one electron with each other. This allows electrons to move about both atoms and gives both atoms access to two electrons; they become a stable H2 molecule joined by a single covalent bond.
Covalent bonding in a molecule of ammonia
Each hydrogen atom needs one more electron to complete its valence energy shell. The nitrogen atom needs three more electrons to complete its valence energy shell. Therefore, three pairs of electrons must be shared between the four atoms involved. The nitrogen atom will share three of its electrons so that each of the hydrogen atoms now has a complete valence shell. Each of the hydrogen atoms will share its electron with the nitrogen atom to complete its valence shell.
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overlap between inner orbitals with an unpaired electron forms a covalent bond between two atoms, the repulsion between two valence orbitals with an unpaired electron prevents bond formation, the overlap between two valence orbitals with an unpaired electron forms a ionic bond between two atoms, and overlap between two valence orbitals with an unpaired electron forms a covalent bond between two atoms