What are Lewis Dot Structures? Why are they Important?

Lewis Dot Structures

Lewis introduced an article, ‘The Atom and the Molecule.’ It explained the idea of forming chemical bonds by sharing electrons with the concept of electron dot structure. The following article describes more about the ‘Lewis Dot Structures.’

Several attempts are made to explain the chemical bond formation in terms of electrons. In 1916 a satisfactory explanation was declared when Lewis and Kössel succeeded in this experiment independently. They provided some logical descriptions by experimenting with the inertness of noble gases.

Lewis’s Approach to Chemical Bonding

Lewis, an American chemist, took the atom as a positively charged ‘Kernel’. This ‘Kernel’ includes an atom’s nucleus and inner electrons. He assumed that the outer shell of the ‘Kernel’ accommodates a maximum of eight electrons.

He supposed that these eight electrons inhabited the corners of a cube surrounding the ‘Kernel’. This way, a single valence shell electron of an atom (suppose sodium) would occupy one corner of the cube. In the occurrence of a noble gas, all the cube’s eight corners are occupied.

Due to the arrangement of eight electrons, it is termed the octet of electrons. This octet of electrons acts for a specifically stable electronic order. Lewis hypothesized that atoms achieve a stable octet because of the linkage of chemical bonds.

To explain his hypothesis, he took the example of sodium and chlorine atoms. In the illustration of sodium and chlorine, this bonding can happen by transferring an electron from the sodium atom to the chlorine atom. As a result, they form Na⁺ and Cl⁻ ions.

In the case of other molecules like Cl₂, H₂,  F₂, etc., the bond is formed by sharing an electron pair between the atoms. In this process, each atom accomplishes a stable outer octet of electrons.

Lewis Structures

Lewis structures are generally called by many similar names as Lewis dot diagrams, electron dot structures, Lewis dot structures, and Lewis electron dot structures. All these names cite only one structure, i.e., the chemical combination between the atoms or molecules.

A Lewis structure is a constructional representation of electrons in a molecule or atom where dots reveal the position of electrons around the atoms, and dot pairs or lines act as the covalent bonds among atoms.

Drawing Lewis structures recognizes the lone pair of electrons in the molecules to help form chemical bonds. Lewis dot diagrams can also form for molecules containing coordination compounds and covalent bonds. The reason is that electrons are split in a covalent bond.

How to Draw Lewis Structures?

For a neutral molecule or an atom, to draw a Lewis dot diagram, follow the given steps:

1. Calculate the valence electrons present at each atom in the given molecule. For instance, in carbon dioxide (CO₂), each carbon consists of four valence electrons. In comparison, oxygen consists of six electrons in its valence shell.
2. Suppose any molecule consists of more than one type of atom. In that case, the least electronegative atom goes in the center.
3. Arrange electrons in such a way that each atom can contribute one electron to form a single bond among each atom.
4. Count the electrons present around each atom. The octet is considered complete if each atom has eight electrons or an octet. If not, proceed to the next step.
5. If the octet is incomplete, redraw the structure to make definite electrons form pairs to attain the number of electrons on each atom to eight.

Examples of Lewis Dot Diagram

The Lewis dot diagrams of some molecules are given below:

Example 1: Explanation of Lewis structure of CO₂ molecule.

In the case of carbon dioxide (CO₂), the initial structure consists of seven valence electrons linked with each oxygen atom and six valence electrons for the carbon atom. The ultimate structure puts two electron pairs or sets of two dots on both oxygen atoms. Two oxygen electron dots face the carbon atom, and two pairs of carbon dots or two electrons are on each side. Four electrons are distributed among each oxygen and carbon, which are figured out as double bonds.

 

Example 2: Lewis structure of CO molecule.

Step 1: Calculate carbon and oxygen atoms’ total valence shell electrons.

The valence shell configuration of carbon is 2s² 2p² and the oxygen atom is 2s² 2p⁴.

Therefore, the total valence shell electrons available = 4 + 6 =10.

Step 2: The framework structure of CO is written as C O

Step 3:

• Draw a single bond, i.e., one shared electron pair, among C and O and fulfill the octet on O.
• The other two electrons act as lone pairs on C. It does not complete the carbon octet. Hence, it is utilized for numerous bonding between C and O atoms.
• In the case of CO, three bonds are required to make it stable. It also pleases the octet rule situation for both atoms.

Conditions to Draw Lewis Structures in Covalent Compounds

The Lewis dot diagrams can also draw for those molecules that consist of identical or different atoms. The chief conditions required are :

• Each bond is established due to the splitting of an electron pair among the atoms.
• Every combining atom donates at least one electron to form a bond among atoms.
• The combining atoms accomplish the valence shell noble gas electronic arrangement, i.e., complete octet state, resulting from the distribution of electrons.
• Therefore, in the case of water and carbon tetrachloride molecules, the formation of covalent bonds with the help of the Lewis dot diagram is given below:

 

Why Are Lewis Structures Important?

Lewis dot diagram provides an idea about the number of atoms participating in the bond formation. It allows

you to know how the atoms will be bonded with each other to form bonds. This knowledge ultimately allows you to understand the shapes and geometries of molecules and their chemical properties.

Applications of Lewis Dot Diagrams

There are many applications of Lewis Dot Diagrams.

• Lewis structures are used to analyze and understand chemical bonds and atomic structures.
• Lewis dot diagrams aid in knowing the type of bonds formed between atoms utilizing valence electrons or/ and lone pairs and how many bonds an atom can or will form with other atoms.
• Lewis structures are useful in predictive analysis. These diagrams assist scientists and students in predicting the bond length, oxidation states, bond strength, the shape of molecules, etc.
• Lewis structures make it easier for the students to study and know more about chemical bonds, reactivity with other atoms, bond formation, and the geometry of molecules pre and post-bond formation.

Advantages and Disadvantages of Lewis Structures

Advantages:

• It is the straightforward and most fundamental theory on an electronic structure where electrons are merely presented as ‘dots’ or for bonding electrons as a straight line among the two atoms. The goal is to obtain the complete electron configuration, i.e., the octet rule and formal charges must be satisfied.
• While the complete structure is more useful to the inexperienced, the simplest is quicker to draw and still conveys the same information to the experienced.
• Different ways to form the ‘same’ Lewis structure depend on what you want to show.

Disadvantages:

• In its simplicity of use, the Lewis dot diagrams do not attempt to describe how the bonds form, the geometry of molecules, or how the electrons are split between the atoms.
• You must learn to recognize any possible Lewis structures and how they can be drawn.
• The ‘best’ Lewis structure, where octets must be complete and formal charges must be lowest, shows only the net charge on the ion in place of all of the formal charges.

Conclusion

Simply speaking, Lewis dot diagrams or Lewis structures are the electronic representation of elements that explain how the electrons are paired to form molecules. From the above discussion, it can be said that the Lewis dot diagram helps in viewing the electrons and lets you determine whether atoms are following the octet rule or not.

With the Lewis dot diagram, you can also see that molecules can have bonding electrons, which are splitting between atoms and non-bonding electrons. These bonding electrons are also known as lone pairs.

As a result, the Lewis dot structure concepts are briefly covered in this article along with a few instances. 

Practice Questions 

Q1 Which of the following anion’s number of bonds surrounding the central atom exceeds four according to the Lewis dot structure? 

• none of the above 
• CO32− 
• NO3− 
• CO43− 

Q2. Draw Lewis dot structures for an atom of each of the following elements: 

• K  
• Si  
• Ar  
• As 

Frequently Asked Questions

1. What are the Lewis symbols?

A: In the formation of a molecule, in chemical combination, only valence electrons take part. Valence electrons are the electrons present in the outer shell of an atom. Generally, inner electrons do not take part in this combination process.

Lewis introduced some simple annotations to represent valence electrons in an atom. These notations or codes are called Lewis symbols. For instance, the Lewis structures using Lewis symbols for the different atoms are given below:

2. What is the Octet Rule?

A: According to Lewis’s electronic theory of chemical bonding, atoms can unite either by sharing valence shell electrons from one atom or element to another or by transfer of electrons to gain a complete octet in their outermost or valence shells. This transfer or sharing of electrons is known as the octet rule. The transferring of valence electrons can be done by gaining or losing electrons.

3. What is a Formal Charge?

A: The formal charge of an atom present in a polyatomic ion or molecule may be described as the variance between the number of valence electrons in a free or isolated state and the electrons’ number allocated to that atom in the Lewis structure.

The formal charge (F.C.) on an atom in a Lewis structure = [net valence electrons in the free atom] – [net non-bonding or lone pair electrons] – (1/2) [net bonding or shared electrons].

4. What are Lewis Dot Structures?

A: The electrical structures of the elements, especially the electron pairings, are reflected in the Lewis dot structures.