Of all the electrons that form part of an atom, the valence electrons are the only ones that participate when atoms combine to form a bond. In ionic bonding, electrons are transferred (either lost or gained), whereas in covalent bonding, two electrons are shared between the two atoms. Double or triple bonds are formed when atoms share more than one electron pair. The atoms do so to attain the stable octet configuration of 8 electrons in their valence shell.
Lewis Structures represents the covalently bonded molecules by exposing these valence electrons as dots (.), one dot for one electron. The dots are used to represent both bond-forming and non-bond-forming electrons (also called the lone pair).
By looking at the Lewis structures, one can fully understand the electron distribution of the molecule.
Lewis structures are also known by different names, such as Lewis dot diagrams, Lewis dot formulas, Lewis dot structures, and Electron dot structures.
Steps to Draw a Lewis Structure - Example of C2H2
1) Calculating the Valence Electrons
Valence Electron of Carbon is 4, and Hydrogen is 1. Therefore, for two Carbon and Hydrogen atoms, the total valence electron count is-
C2H2= 2 x 4 + 2 x 1= 10
2) Obtaining the electron pairs
Obtaining the electron pair requires dividing the total valence electrons by two, as shown below,
Valence Electrons/ 2= 10/2 = 5
There are 5 electron pairs in the C2H2 molecule.
3) Assigning the single bonds
A carbon atom will bond to another carbon atom to form a carbon chain. Since the molecule C2H2 has only two carbon and hydrogen atoms, the structure can be drawn as -
H-C-C-H
4) Deducting the single bonds from the electron pairs
Since we used three single bonds (or three electron pairs) the remaining electron pairs are two.
5-3= 2
5) Assigning the Lone pairs
The two remaining electrons will be shown as the lone pairs on the most electronegative atom, carbon. Now each carbon atom has 6 valence electrons.
6) Sharing the electrons to complete the octet
To complete the octet, the lone pairs are shared between two carbon atoms, forming three bonds. Now each carbon atom has 8 valence electrons.
