The correct option is Langmuir.
Explanation
The electronic theory of valency developed through the contributions of several scientists in the early 20th century. While the mechanism of electron sharing was identified early on, the specific terminology used to classify these bonds was introduced subsequently to refine the theory.
Detailed Analysis:
- Langmuir Langmuir: Correct. Irving Langmuir refined the static atom model proposed by G.N. Lewis. In his 1919 paper, "The Arrangement of Electrons in Atoms and Molecules," Langmuir introduced the term "covalence" (later covalent) to describe the number of electron pairs an atom shares with its neighbors.
- Lewis Lewis: Incorrect. G.N. Lewis is credited with proposing the concept of the shared electron pair bond in 1916 (Lewis structures). However, he did not use the term "covalent" in his original work; he referred to the phenomenon simply as the sharing of electron pairs.
- Kossel Kossel: Incorrect. Walther Kossel (1916) is primarily associated with the theory of electrovalent (ionic) bonding. His work focused on the transfer of electrons and the formation of ions to achieve stable noble gas configurations, rather than electron sharing.
- Gillespie Gillespie: Incorrect. Ronald Gillespie is associated with the development of the VSEPR (Valence Shell Electron Pair Repulsion) theory in 1957, which explains molecular geometry. This occurred decades after the terminology for covalent bonding was established.
Key Takeaway: While G.N. Lewis discovered the principle of the shared electron pair, Irving Langmuir is responsible for coining the term 'covalent' to describe this type of bond.