The development of periodic table and concept of electronic configuration gave chemists a rationale for molecule and compound formation.
This explanation formulated by Gilbert Lewis is that atoms combine in order to achieve a more stable electronic configuration. Maximum stability results when an atom is isoelectronic with a noble gas. When atoms interact to form a chemical bond, only their outer regions are in contact, for this reason, when we study chemical bonding, we are concerned primarily with the valence electrons of the atoms. To keep track of the valence electrons in a chemical reaction and to make sure that the total number of electrons does not change, chemists use a system of dots devised by Lewis called Lewis dot symbols.
Lewis structures (also known as Lewis dot diagrams, electron dot diagrams and electron dot structures) are diagrams that show the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule.
In Lewis structure, the dots representing the valence electrons which are placed on four sides of the atomic symbol–the top, the bottom and to the right and left side of the chemical symbol represent the position of the element in the periodic table of elements. For example, oxygen and sulfur atoms have six valence electrons in their valence shell representing that they are placed in the 6th group in a periodic table. Similar is with other elements too.
Covalent bonds share electrons in order to form a stable octet around each atom in the molecules. Hydrogen is the exception it only requires 2 electrons (a duplet) to be stable. According to Lewis, the elements undergo bonding in order to attain the stable noble gas configuration (i.e., 8 valence electrons in their outer most shell) of the nearest elements in a periodic table. This observation led to the guideline known as octet rule.
Atoms tend to gain, loose, or share electrons until they are surrounded by 8 valence electrons. In terms of Lewis symbols, an octet is thought as four pair of valence electrons arranged around the atom. For example, Neon (10), with 8 electrons in its valence shell. During bond formation, in order to obtain the octet rule, electrons are shared between two atoms, each donating one electron. The shared pair of electrons represents a covalent bond represented by a single bond or line in Lewis structure. If more than one pair of electrons are shared between two atoms, such covalent bonds are known as multiple bonds.