Question

Who proposed valence bond theory?

Answer 1

Out of the many theories that were given by various scientists to explain the nature and need of atoms to form different types of bonds, the valence bond theory was a very useful theory that highlighted the purpose of bond formations by different atoms.

Complete answer:
The valence bond theory is based on the idea that the electrons present in the outermost shell of an electron (valence electrons) are available for sharing and donations and can therefore be used to interact with other neighboring atoms so as to form bonds.
The theory utilizes the Lewis concept of electron paired bonds and is based on the assumption that the orbitals associated with atoms that hold the electrons within them somehow merge or overlap with each during bond formation.
The driving force for the atoms to overcome electron-electron repulsions and bring their orbitals close enough to allow an overlap is that each element attempts to gain the nearest noble gas configuration. Electron rich and electron deficient species are unstable and unsatisfied and therefore bond formations are accompanied by release of energy.
This theory also talks about hybridizations that involve mixing of orbitals to give new and better orbitals of similar sizes and energies that overlap efficiently. This concept also helps explain the structure and geometry of different molecules based on the interaction of the electrons present on the atoms.
$\Rightarrow$ This theory called the valence bond theory was given by the American scientists Linus Pauling and John C. Slater.

Note:
Despite its usefulness and applications, valence bond theory was outdated and replaced by the molecular orbital theory that incorporates quantum mechanical calculations and is believed to provide an accurate and true picture of the bonding situation in between atoms.