Lewis Structures: As valence electrons are significant to an atom’s reactivity, it is essential to represent it by simple diagrams. Lewis Structures are pictorial representations of molecules in which the valence electrons present in an atom are represented as dots. Hence, these structures are also known as electron dot diagrams. 

The Lewis structure was named after Gilbert N. Lewis, who introduced it in his 1916 article called ‘The Atom and the Molecule’. Lewis dot structures are extensions of the concept of the electron dot diagram. This structure can be drawn for any covalently bonded molecule and coordination compound. Read on to learn more exciting facts about Lewis structures and how to draw SO2 Lewis Structure, NH3 Lewis Structure, CO2 Lewis Structure and more.

Lewis Structure – Introduction

Atoms are made up of neutrons, protons \((+)\) and electrons \((-)\). The nucleus of an atom is made up of positive protons and neutral neutrons. Electrons are found moving around the nucleus in energy shells. The shells can be numbered numerically as \(1,2,3…\) so on or by using letters like \({\rm{K}},{\rm{L}},{\rm{M}}\) so on.

This means the energy shell K is denoted as \(1\) and has the lowest energy. The orbits are arranged in increasing energy levels, with the lowest energy shell being closer to the nucleus and the highest energy shell being farthest from the nucleus.

What is Lewis Dot Structure?

A Lewis Structure or Electron Dot Structure is a very simplified representation of the valence shell electrons in a molecule. It denotes the way the valence electrons are arranged around the individual atoms in a molecule.

The Lewis structure was named after Gilbert N. Lewis, who introduced it in his \(1916\) article “The Atom and the Molecule.”

In a Lewis Structure, electrons are represented as “dots” surrounding the central metal atom.  The central metal is denoted by using its chemical symbol from the Periodic Table.

In Lewis Structures, a line is used to represent the bonding electrons between two combining atoms.  The Lewis Structure also denotes the number of lone pairs of electrons present around the central atom.

Lewis Structures can be drawn for ionic, covalent and coordination compounds. However, we can draw it only if we know the molecular formula of the compound. It is also helpful to represent the atomic state of elements. 

How to Draw Electron Dot Formula?

We represent an electron dot structure by using the symbol of an element. The dots that represent the valence electrons are added to the chemical symbol of an element in a clockwise manner. We cannot put the dots anywhere around the symbol. Instead, we need to imagine a square around the chemical symbol. The dots should be neatly drawn on the four sides of the square with no more than two electrons on each side.

1. The maximum number of dots can be eight. This is because the maximum number of valence electrons can be only eight, thereby satisfying the octet rule. 
2. The octet rule is significant to a Lewis dot structure. The octet rule states that atoms tend to gain, lose, or share valence electrons to attain noble gas configuration, which imparts stability to the atom. Only the s and p electrons are involved in the octet rule; the d and f electrons are not considered. 

The steps needed to draw them are:

Step 1: Count all the Valence Electrons

The total number of valence electrons present in the molecule of the compound is calculated by adding the individual valence electrons of each atom.

Step 2: Determine the Central Atom

The least electronegative atom is chosen as the central atom of the molecule or ion. The central metal atom is the one to which all the other atoms will be bonded. The central atom to be chosen should possess the least subscript in a given molecule.

Step 3: Draw all the Single Bonds to the Central Metal Atom

A single bond represents \(2\) valence electrons, one from each atom contributing to bond formation. A line represents a single bond.

Step 4: Calculation of lone Pairs of Electrons

The number of valence electrons used for bonding in step \(3\) is subtracted from the total number of valence electrons calculated in step \(1\). The remaining electrons are assigned to each atom as lone pair of electrons.

Step 5: Satisfying Octet Configuration for each of the Participating Atoms

Turn all lone pairs into double or triple bonds to satisfy the octet configuration of each combining atom.

1. For an anion, we need to add extra electrons to the dot structure. The number of an extra electron that is to be added is always equal to the charge on the anion.
2. For a cation, we need to subtract electrons from the total count. The number of extra electrons that is to be subtracted is always equal to the charge on the cation.

Illustration To Draw Electron Dot Formula

Let us take the example of Sulphur trioxide \({\rm{S}}{{\rm{O}}_{\rm{3}}}\)

Step 1 – Calculation of the total number of valence electrons in sulfur trioxide molecule.

Both oxygen and Sulphur belong to group \(16\) of the periodic table. Hence, Sulphur and oxygen contain \(6\) valence electrons each. 

So in \({\rm{S}}{{\rm{O}}_{\rm{3}}}\) we have \({\rm{S = 6 \times 1 = 6}}\) valence electrons

\({\rm{O = 6 \times 3 = 18}}\) valence electrons

Hence, there are a total of \(24\) valence electrons in \({\rm{S}}{{\rm{O}}_{\rm{3}}}\) molecule.

Step 2 – Determination of the central metal atom.

In \({\rm{S}}{{\rm{O}}_{\rm{3}}}\), Sulphur being less electronegative than oxygen, it is chosen as the central metal atom. Also, Sulphur has the lowest subscript. We can represent sulfur trioxide as below-

Step 3- Draw single bonds to the central atom.

Step 4 – Calculation of lone pairs of electrons.

In total, we had \(24\) valence electrons as calculated in Step 1. There are three single bonds, so we have used \(3\) pairs or \(6\) valence electrons out of \(24\). The remaining valence electrons are \(18\) in the count. These remaining \(18\) valence electrons are used as lone pairs on the atom. This is as shown below-

Step 5 – Satisfying the octet configuration for each of the participating atoms.

In \({\rm{S}}{{\rm{O}}_{\rm{3}}}\), the Sulphur atom does not have an octet configuration. It has only \(6\) valence electrons. So, to complete the octet configuration of Sulphur, one of the three oxygen atoms shares one lone pair of electrons with the Sulphur atom. This results in the formation of a double bond.

Steps to Draw Lewis Structure

The steps to draw the Lewis structures of various types of compounds are given below:

Lewis Structure of oxygen \(\left( {{{\rm{O}}_{\rm{2}}}} \right)\)

Oxygen belongs to group \(16\) of the Periodic Table. Hence, oxygen has \(6\) valence electrons.

Step 1 – Calculating the total valence electrons.

In \({{\rm{O}}_2}\), total number of valence electrons \(= 6 × 2 = 12\) valence electrons

Step 2 – Determination of the central metal atom.

In \({{\rm{O}}_2}\), there are only two oxygen atoms. Hence, we can choose any one of the oxygen atoms as the central metal atom.

Step 3 – Connecting the participating atoms through single bonds.

In \({{\rm{O}}_2}\), \(2\) of the oxygen atoms are connected through single bonds. The structure of the \({{\rm{O}}_2}\) molecule in Step \(3\) is as shown below –

Step 4 – Calculation of lone pair of electrons.

In \({{\rm{O}}_2}\),

Total number of valence electrons \(= 6 × 2 = 12\)  valence electrons

Valence electrons used in making \(1\) single bond \(= 1 × 2 = 2\) valence electrons

Hence, total number of valence electrons left after making a single bond \(= 12 – 2 = 10 \)valence electrons.

These \(10\) valence electrons are distributed over the \(2\) oxygen atoms. This is as shown below-

Step 5 – Satisfying the octet configuration for each of the participating atoms.

In \({{\rm{O}}_2}\), one of the oxygen atoms does not possess the octet configuration. Hence, the octet configuration for this oxygen atom is fulfilled by the other oxygen atom. The other oxygen atom shares its lone pair of electrons with the oxygen atom lacking an octet configuration. This results in the formation of a double bond. The Lewis structure for oxygen molecule is as shown below-

Lewis Structure of Carbon Dioxide \(\left( {{\rm{C}}{{\rm{O}}_2}} \right).\)

In \(\left( {{\rm{C}}{{\rm{O}}_2}} \right),\) oxygen belongs to group \(16\) of the Periodic Table, and carbon belongs to group \(14\) of the Periodic Table. Hence, oxygen has \(6\) valence electrons, and carbon has \(4\) valence electrons.

Step 1- In \(\left( {{\rm{C}}{{\rm{O}}_2}} \right)\) we, have \({\rm{C = 4 \times 1 = 4}}\) valence electrons

\(O = 6 \times 2 = 12\) valence electrons

Hence, there are \(16\) valence electrons in \(\left( {{\rm{C}}{{\rm{O}}_2}} \right)\) molecule.

Step 2 – Determination of the central metal atom.

In \(\left( {{\rm{C}}{{\rm{O}}_2}} \right)\), carbon has the lowest subscript. So, carbon is chosen as the central atom. We can represent carbon dioxide as below-

Step 3 – Connecting the two oxygen atoms through single bonds to the carbon atom.

Step 4 – Calculation of lone pairs of electrons.

Total number of valence electrons \(= 16\) 

Valence electrons used in bonding \( = 1 \times 2 = 2(1\) single bond \(=2\) electrons).

Valence electrons remaining \(= 16 – 4 = 12\)

Hence, \(12\) valence electrons are remaining, distributed as lone pairs over the \(2\) oxygen atoms.

Step 5 – Satisfying the octet configuration.

After step \(4\), it was found that the octet configuration of both the oxygen atoms is satisfied, but the octet configuration of the carbon atom is not satisfied. The oxygen atoms possessing lone pair of electrons share it with the neighbouring carbon atomsThis leads to a double bond between the \(2\) oxygen atoms and carbon atoms pairs, as shown below.

Lewis Structure of Carbon Monoxide (CO)

In \({\rm{CO}}\), oxygen belongs to group \(16\) of the Periodic Table, and carbon belongs to group \(14\) of the Periodic Table. Hence, oxygen has \(6\) valence electrons, and carbon has \(4\) valence electrons.

Step 1- Calculating the total number of valence electrons.

In \({\rm{CO}}\) we have-  \({\rm{C = 4 \times 1 = 4}}\) valence electrons

 \({\rm{O = 6 \times 1 = 6}}\) valence electrons

Hence, there are \(10\) electrons in \({\rm{CO}}\) molecule.

Step 2 – Determination of the central metal atom.

In \({\rm{CO}}\), both carbon and oxygen have the lowest subscript. The carbon atom is less electronegative than the oxygen atom. Hence, carbon is chosen as the central atom. We can represent carbon monoxide as below-

Step 3 – Connecting the participating atoms (C and O) through a single bond.

Step 4 – Calculation of lone pairs of electrons.

Total number of valence electrons \(= 10\) 

Valence electrons used in bonding \( = 1 \times 2 = 2(1\) single bond \(=2\) electrons)

Valence electrons remaining \(= 10 – 2 = 8\)

Hence, \(8\) valence electrons are remaining, distributed as lone pairs over the oxygen atom and carbon atom.

Step 5 – Satisfying the octet configuration.

After step \(4\), it was found that the octet configuration of the oxygen atom is satisfied but for the carbon atom, it is not satisfied. The oxygen atom possessing \(3\) lone pair of electrons shares two lone pairs with the neighbouring carbon atom. This leads to a triple bond between the carbon atom and oxygen atom, which is shown below.

Lewis Structure of Ammonia \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\)

In \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\), nitrogen belongs to group \(15\) of the Periodic Table, and Hydrogen belongs to group \(1\) of the Periodic Table. Hence, nitrogen has \(5\) valence electrons, and hydrogen has \(1\) valence electron.

Step 1- Calculating the total number of valence electrons.

In \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\), we have \({\rm{N = 5 \times 1 = 5}}\) valence electrons

\({\rm{H = 1 \times 3 = 3}}\) valence electrons

Hence, there are \(8\) valence electrons in \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\) Molecule.

Step 2 – Determination of the central metal atom.

In \(\left( {{\rm{N}}{{\rm{H}}_{\rm{3}}}} \right)\), nitrogen has the lowest subscript. Hence, it is chosen as the central metal atom. We can represent ammonia as below-

Step 3 – Connecting the participating atoms (N and H) through a single bond.

Step 4 – Calculation of lone pairs of electrons.

Total number of valence electrons \(= 8\) 

Valence electrons used in bonding \(= 3 × 2 = 6\) (\(3\) single bond \(= 6\) electrons)

Valence electrons remaining \(= 8 – 6 = 2\)

Hence, \(2\) valence electrons are remaining, distributed as lone pairs over the nitrogen atom.

Step 5 – Satisfying the octet configuration.

After step \(4\), it was found that the octet configuration of the nitrogen atom and hydrogen atom is satisfied. As the octet configuration of all the participating atoms that is nitrogen and hydrogen is satisfied, there is no need for extra bond formation. The Lewis structure of ammonia is as shown below-

Formal Charge

The formal charge on an atom in a molecule reflects the electron count associated with the atom compared to the isolated neutral atom. If the atom has given away electrons, it will be \(+ve\), and if it has gained electrons, it will be \(-ve\).

No Lewis structure is complete without the formal charge. 

Significance of Lewis Structures

1. The goal is to obtain the “best” electron configuration, i.e. the octet rule and formal charges need to be satisfied.
2. It defines the nature of the bond and position of atoms of the molecule which are connected in the molecule.
3. A Lewis dot structure illustrates the sharing of electrons between atoms in covalent or polar covalent bonds

Summary

To conclude, we can say that Lewis Structures are of great importance in explaining the arrangement of outer shell valence electrons of an atom or a molecule. Lewis structures help us to visualize the number of valence electrons present around an atom or a molecule. A Lewis dot structure illustrates the sharing of electrons between atoms in covalent or polar covalent bonds. It defines the nature of the bond and position of atoms of the molecule which are connected in the molecule. The central atom is usually the atom with the lowest subscript in the molecular formula and the atom that can form the most bonds. It is also the atom having low electronegativity. If all of the atoms usually form the same number of bonds, the least electronegative atom is usually the central atom. In addition, it gives us an idea about the bond type and the lone pair of electrons present over the participating atoms. In this article, we studied what are the weird lines and dots surrounding an atom and how they are useful in chemistry.

FAQs about Lewis Structure

The most commonly asked questions about Lewis structure are answered here:

Q.1. How do you know where to put the dots on a Lewis structure? Ans: The dots representing the valence electrons are added to the chemical symbol of an element in a clockwise manner. Therefore, we cannot put the dots anywhere around the symbol. Instead, we need to imagine a square around the chemical symbol. The dots should be neatly drawn on the four sides of the square with no more than two electrons on each side.

Q.2. What do Lewis structures show? Ans: Lewis structures help us to visualize the number of valence electrons present around an atom or a molecule. In addition, it gives us an idea about the bond type and the lone pair of electrons present over the participating atoms.

Q.3. What are the limitations of Lewis Structures? Ans: The limitations of Lewis structure are as follows- 1. Lewis structures do not account for the aromaticity of the compound. 2. Lewis structures do not hold good for the resonance structures. 3. Lewis structures do not explain the magnetic behaviour of compounds. 4. These structures do not account for the shape and geometry of the molecules.

Q.4. What is the central atom in a Lewis structure? Ans: The central atom is usually the atom with the lowest subscript in the molecular formula and the atom that can form the most bonds. It is also the atom having low electronegativity. If all of the atoms usually form the same number of bonds, the least electronegative atom is usually the central atom.

Q.5. What is the significance of Lewis structures? Ans: A Lewis dot structure illustrates the sharing of electrons between atoms in covalent or polar covalent bonds. It defines the nature of the bond and position of atoms of the molecule which are connected in the molecule.

Learn Formal Charge In Lewis Structure

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