The Rutherford model of the nuclear atom unveiled the atom's exterior is occupied by the electrons, implying that electrons are responsible for atom contact and chemical bond formation. Gilbert N. Lewis defined valence and chemical bonding in 1916 by stating that atoms have a tendency to form a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding produces octets by sharing electrons, whereas ionic bonding produces octets by transferring electrons from one atom to the next.The terminology "covalence" was coined by Irving Langmuir in 1919, who noted, "The number of pairs of electrons that any particular atom shares with neighbouring atoms are called the covalence of that atom." As the word co- implies "together," a covalent connection implies that the atoms have the exact valence. Following that, it is now more customary to speak of covalent bonds rather than valence, which has lost favour in the higher-level study due to developments in chemical bonding theory but is still extensively employed in introductory studies as a heuristic introduction to the subject.