Covalent bond

Lewis (1919) proposed that some atoms instead reach a stable noble-gas configuration by SHARING one or more electron pairs with another atom, rather than by transferring electrons outright — each atom contributes one electron to the shared pair and has an equal claim on it. Langmuir named this the covalent bond (so the whole idea is often called the Lewis-Langmuir concept), conventionally drawn as a single dash (—) between the bonded atoms. Formally, a covalent bond is the attractive force arising from mutual electron sharing between two atoms of similar electronegativity, or with only a small electronegativity difference between them. In H2, each hydrogen atom ($1s^1$) needs one more electron to complete its valence shell; when two H atoms approach and share their single electrons, both attain the helium-like duplet, giving H:H or H—H. In Cl2, each chlorine atom ($[Ne]3s^23p^5$) is one electron short of argon's configuration; sharing one pair between the two Cl atoms (Cl—Cl) lets both attain a full octet. The same sharing idea extends to molecules built from different atoms, such as CCl4 (where carbon and all four chlorines end up with a complete octet) and H2O (where both hydrogens reach a duplet and oxygen reaches an octet). Where two atoms share TWO electron pairs the resulting bond is …