Who gave the concept of a covalent bond? What is a covalent bond? Explain with one example.

(N/A) Langmuir $(1919)$ refined the Lewis postulations by abandoning the idea of the stationary cubical arrangement of the octet and by introducing the term covalent bond.
According to the Lewis-Langmuir theory,atoms can combine by sharing electrons between them. This type of bond is called a covalent bond,and each atom contributes an equal number of electrons for sharing in order to acquire the configuration of the nearest noble gas.
Example: Covalent bond in a dichlorine $(Cl_{2})$ molecule.
The $Cl$ atom with electronic configuration: $[Ne] \ 3s^{2} 3p^{5}$,is one electron short of the argon noble gas configuration.
The formation of the $Cl_{2}$ molecule can be understood in terms of the sharing of $1$ pair of electrons between the two chlorine atoms,each chlorine atom contributing one electron to the shared pair.
The electron pair takes part in the octet of both $Cl$ atoms.
So,both chlorine atoms get an octet like $Ar$: $[Ne] \ 3s^{2} 3p^{6}$ or $[Ar]$.
Note: '$\circ$' and '$x$' symbols are used to represent electrons of both chlorine atoms. '-' is used to represent the covalent bond between two $Cl$ atoms.