Lewis Electron Dot Structures: Steps, Examples and Limitations

Lewis electron dot structures are the diagrammatic representation of covalently bonded atoms in a molecule. They also show lone pairs of electrons that may exist in the molecule. They are used in the case of covalently bonded molecules as well as coordination compounds. This system was introduced by Gilbert Lewis in 1916. Hence the diagrams are called Lewis structures or Lewis electron dot structures or electron dot arrangement.

Read More: Lewis acids

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Lewis Electron Dot Structures

A covalent bond involves valence electrons of atoms interacting with each other. Gilbert Lewis proposed simplified notations taking into account the role of valence electrons. This representation involves representing the atom by its chemical symbol and valence shell electrons by dots around the atomic symbol. Thus it gives a simplified picture of the overall chemical bond.

Lewis Electron Dot Structure

An atom is represented by its chemical symbol. Lines representing bonds between atoms are drawn. Lone pairs are drawn as dots. Instead of a line to represent a bond a pair of dots (2 electrons) can also be used. By placing dots or crosses around the chemical symbol of the atom valence shell electrons are represented. One thing must be noted: one dot = one electron, two dots = Bond. However, to represent a bond in most cases straight lines are used.

CO₂ Lewis Dot Diagram

This is the Lewis electron dot structure for Carbon dioxide.

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Steps to Draw Lewis Electron Dot Structures

Following steps are followed to draw lewis structures:

Step 1: For the molecule under consideration, calculate the total number of valence electrons, for this add up the valency of all the atoms in the molecule. This gives the total number of electrons for the Lewis structure.

Step 2: For an anionic molecule add one electron for one negative charge to the valency of the respective atom. Similarly, for cations subtract one electron for one positive charge from the valency of the respective atom.

Fluorine with Valence Electrons

Step 3: Keeping the most electronegative atom in the centre, single bond pairs are assigned first.

Step 4: After assigning the single bond pairs, lone pairs and multiple bond pairs are distributed in accordance with the octet rule.

Formation of Covalent Bond

Step 5: According to the octet rule, each atom which is in the bond must have 8 electrons and for this criteria, a lone pair can be converted to a bond sharing pair.

Fluorine Molecule

Step 6: For molecules where more than one lewis structure exists then the one with the least formal charge is selected. The formal charge can be checked by the formula: 

FC = V - L - ½ S

Where, 

FC → Formal Charge on the atom

V → Total number of electrons 

L → Total number of lone pairs

S → Total number of shared electrons.

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Examples Of Lewis Electron Dot Structures

Lewis Electron Dot Structure for O₂ molecule

• An oxygen atom has 6 valence electrons in the valence shell and so it needs 2 more to complete the octet.
• So both the atoms contribute two atoms each for the bond. Hence a double bond is formed.

Lewis Electron Dot Structure for the molecule: CO₂

• An oxygen atom has 6 valence electrons and a carbon atom has 4.
• So carbon shares 2 with one oxygen atom and 2 with the other oxygen atom. Hence two double bonds are formed.

Lewis Electron Dot Structure of the molecule: CO (carbon monoxide)

• The carbon atom has a valency of 4. Hence it needs 4 more to complete the octet.
• The bonding oxygen atom needs two more to fill its octet.
• So to satisfy the octet structure we get a triple bond between C and O atoms.

Lewis Electron Dot Structure for the molecule: HCOOH (Formic Acid)

• In the first step use four electron pairs to form single bonds between neighbouring atoms.

• Except for hydrogen, complete the octet of each atom by adding two lone pairs on each oxygen atom and one oxygen atom gets double-bonded between C and O.

Lewis Electron Dot Structure for the molecule: NO₂^- (nitrite ion)

• First, count all the valence electrons of one nitrogen and two oxygen atoms and the additional one negative charge for which we will add 1 electron (anionic case).
• So total valence electrons = 5 + 12 + 1 = 18.
• First, draw single bonds between the atoms. Try to complete the octet of electrons on all the atoms. For nitrogen to have its octet complete we give a double bond between N and one of the two oxygen atoms.

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Limitations Of Lewis Electron Dot Structures 

• Lewis structures do not give any idea about the release of energy during the formation of a covalent bond.
• The same molecule can have different lewis structures thus leading to ambiguity. 
• Lewis structures do not give any information about the geometrical shapes of molecules.
• No information could be known about bond angles and bond lengths.
• The lewis electron dot structures do not give any information about lone pair-bond pair interaction.

Read More: Thermochemistry

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Things To Remember

• For molecules where more than one lewis structure exists then the one with the least formal charge is selected. The formal charge can be checked by the formula: FC = V - L - ½ S
• Lewis electron dot structure representation involves representing the atom by its chemical symbol and valence shell electrons by dots around the atomic symbol.
• They are used in the case of covalently bonded molecules as well as coordination compounds.
• The lewis electron dot structures do not give any information about lone pair-bond pair interaction.
• According to the octet rule, each atom that is in the bond must have 8 electrons and for this criteria, a lone pair can be converted to a bond sharing pair.
• Lewis structures do not give any information about the geometrical shapes of molecules.

Read More: Structure of Glucose and Fructose