The first time the use of two dots or a colon (:) to represent two electrons acting as connecting links between the two atoms was proposed by G.N. Lewis in 1916.
In his landmark paper, ‘the atom and the molecules,’ he attempted to draw molecular structures using dots to represent the covalent nature of the chemical bonds, now popularly known as the Lewis structures.
So, a molecule of Cl2 was Cl : Cl, two electrons connecting two Cl atoms.
Therefore, the Lewis structure uses dots for two electrons to show the bonding or sharing of the valence electrons in covalently bonded molecules.
Lewis referred from the previous works of Parson and Abegg that the most stable condition for the atomic shell is the one with 8 electrons, a theory now known as the octet rule.
By sharing, all atoms try to attain a stable state of an octet or a duplet configuration (also called the noble gas configuration) of their nearest inert gas; two electrons for Hydrogen (He gas configuration) and an octet for the second-row elements (Ne gas configuration).
Therefore, Lewis structures show how the atoms in a molecule are linked, whether any atom possesses nonbonding electrons or whether the molecule has any formal charge.
Drawing Lewis Structures
Consider a molecule of methane (CH4)
Carbon has 4 (belongs to group 4 of the periodic table), and hydrogen has 1 valence electron. By sharing its one valence electron with one from the hydrogen, the carbon receives 2 electrons. These two electrons are not confined to any atom but belong to both atoms’ nuclei.
In each case, the Hydrogen atom achieved duplet by sharing its one electron with the carbon, and the carbon attained an octet configuration by sharing its 4 with four Hydrogen atoms.
Nonbonding electrons are those valence electrons that are not shared in bond formation. A pair of nonbonding electrons are often called a lone pair. The Nitrogen, Oxygen, and Halogen atoms have one, two, and three lone pairs in their stable compounds. A correct Lewis structure must show any lone pairs.
Still, Kekule modified Lewis structures are widely used in organic chemistry to draw and explain chemical reactivity.
However, as the size of the compound increases, it becomes difficult to depict it with the help of Lewis structures. Modified Lewis structures are called Kekulé structures where the two-electron dots are replaced with a line (-) depicting a bond.
