Covalent bonds are formed when atoms share electrons, one from each atom in a single bond, to form electron pairs, usually making their outermost shells up to eight electrons by this means. This would make them more stable, less reactive and an electronic structure like a noble gas.
They are most frequently formed between pairs of non-metallic elements. Non-metallic elements usually have from four to eight electrons in their outermost shells, the so-called valency electrons, which are used for chemical bonding. In any given “full” shell of eight electrons, the electrons occur in four pairs, but in incomplete shells, the electrons exist singly where possible.
Sometimes, atoms of elements form covalent bonds with other atoms of the same element. Thus two chlorine atoms form the chlorine molecule, by sharing their unpaired electrons. In the case of oxygen , there are two unpaired valency electrons in each atom, so that two electron-pair bonds are formed between the two atoms to
The term "covalence" in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled "The Arrangement of Electrons in Atoms and Molecules". Langmuir wrote that "we shall denote by the term covalence the number of pairs of electrons that a given atom shares with its neighbors."
The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms. He introduced the Lewis notation, in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds. Multiple pairs represent multiple bonds, such as double bonds and triple bonds. An alternative form of representation, has bond-forming electron pairs represented as solid lines. Lewis proposed that an atom forms enough covalent bonds to form a full outer electron shell....
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