Valence Bond Theory
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The valence bond theory (VB theory) or the molecular orbital theory (MO theory) explains why and how electrons are shared between atoms.
The VB theory imagines individual atoms, each with its own orbitals and electrons coming together and forming covalent bonds of the molecule.
The MO theory looks at the molecule as a collection of positive nuclei surround by electrons occupying sets of molecular orbitals.
A bond between two atoms is formed when a pair of electrons is shared by two overlapping orbitals, according to the VB theory.
For example, in a hydrogen molecule, the two 1s orbitals from each H atoms overlap and share electrons.
When HF form, the 1s orbital in the H and the 2p orbital in the F overlap.
2 2p orbitals can overlap linearly, as in F2.
The examples above are all called sigma () bonds.
They all have only one place where they overlap.