Lewis Dot Structures: Definition, Structure and Sample Questions

The Lewis dot structure was named after the great American chemist Gilbert Newton Lewis. If the Molecular formula of a compound is known, one can draw its electron dot structure or lewis dot structure and can define the nature and position of its bond and molecules respectively. Here we will discuss more about the concept with some solved examples and questions.

Lewis dot structures or electron dot structures describe the chemical bonding amongst atoms inside a molecule with help of diagrams. Each of the atoms inside a molecule has lone pairs inside them, with the help of the lewis dot structure their total number can be displayed. Lewis dot structures are also known as electron dot or Lewis structures. Lewis explained a base and an acid as an electron pair donor and as an electron pair acceptor respectively.

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Definition of Lewis Dot Structure

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Lewis Dot Structure

Lewis dot structures show how the electrons are paired in an element. They help to sum up details about bonding and can also be described as something like electron bookkeeping. The geometry of the molecule can be easily understood with such dot structures. Each electron is represented by a dot in the diagram and a bond is represented by a pair of dots for atoms between chemical symbols. It is basically a visual representation of the electron distribution revolving around an atom. 

Read More: Concentration Cell

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How to Draw Electron Dot Structures?

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Lewis Dot Structure

For every valence electron, a Lewis electron dot formula consists of one dot. Stages to enunciate the electron dot formula are mentioned below: 

1. Display a comprehendible bonding pattern utilizing only the element symbols.
2. Collect valence electrons from all atoms and keep an account of their total amount.
3. Calculate the number of bonds using the N – A = S equation.
4. N is equal to the sum of the number of valence electrons
5. A is equal to the total amount of valence electrons collected aside.
6. S is the total no. of the shared electrons in a molecule.
7. Distribute the collected aside valence electrons to calculate the total bonds.
8. Distribute the rest of the electrons.

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How to Draw Lewis Dot Structure?

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Lewis electron dot structure expresses the bonding atoms, the number of bonds in the molecule and the lone pairs that remain in the bonding atom.
While drawing a Lewis structure, the following steps must be followed:

• Individual valencies of each atom are added up to calculate the total amount of valence electrons present in the molecule.
• Extra electrons are added to the lewis dot structure of the molecule if it appears to be an anion. As added electrons equal the magnitude of negative charges.
• Electrons are subtracted from the total count to make up for the positive charge if the molecule is cationic.
• The central atom is adjusted to be the least electronegative atom of the ion or molecule.
• All the atoms are now connected through single bonds.
• Now, each atom belonging to the molecule is assigned the lone pairs of electrons. Commonly, the most electronegative atoms get paired with these lone pairs of electrons.
• After assigning the lone pairs, the octet valency of each atom must be satisfied with a double or triple bond, but only if every atom does not have an octet configuration
• If required, The octet rule for two atoms can also be satisfied by converting a lone pair into a bond pair.
• Now check which bond has the lowest formal charge, and in turn, you will get the most stable one.
• To calculate the formal charge, use the following formulae accordingly:

1. Formal charge = (Total no. of valence electrons in a free state) - (Total no. of electrons assigned in Lewis structure).

2. Formal charge = (Total number of valence electrons in a free state) - ( Total number of nonbonding pair of electron lone pair) - ½ (total number of bonding electrons).

3. Some of the bonding electrons are shared between two atoms, therefore a Factor ½ is attached to them.

• Furthermore, it should be noted that you must only consider the valence electrons while drawing Lewis dot structures and ignore the electrons that do not belong to the outermost shell.

Read More: Types of Titration

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Examples of Lewis Dot Structure

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1. Lewis Structure of O₂

• There are almost 6 electrons in the valence shell of an atom of oxygen.
• The oxygen atom must participate in two single bonds or one double bond to attain an octet configuration because four of the valence electrons exist in lone pairs.
• The atoms form a double bond because there are only two oxygen atoms in an O₂ molecule, and its Lewis Dot structure looks like this: 

Lewis Structure of O₂

2. Lewis Structure of CO₂

• The central atom of this molecule is Carbon.
• Only one carbon atom is bonded to Oxygen-containing 6 valence electrons which form 2 lone pairs and thus, it must form a double bond.
• Carbon is doubly bonded to each oxygen atom as it has four valence electrons, which results in zero lone pairs.

Lewis Structure of CO₂

3.  Lewis Structure of Carbon Monoxide (CO) 

• There is one carbon atom and one oxygen atom in a molecule of Carbon monoxide.
• The carbon atom requires four electrons to obtain an octet configuration whereas the oxygen atom requires two.
• Therefore, the oxygen atom bonds with a lone pair of donated electrons to satisfy the valency.
• A triple bond connecting a carbon and an oxygen atom with each holding a lone pair of electrons from the Lewis Dot structure diagram given below.

Lewis Structure of Carbon Monoxide

4. Lewis dot structure of the carbonate ion CO₃^2-

• Carbon-oxygen bonds are likely to be equal.
• One electron pair is delocalized over the region of all three carbon-oxygen bonds.
• The electron structure in resonance terms is shown above. 

Lewis Dot Structure of Carbonate Ion CO₃^2-

5. Lewis Dot structure of Cyclopentane.

• Five carbon atoms are connected through single bonds of cyclopentane in a cyclic form. 
• The molecular formula of cyclopentane is C₅H₁₀. 
• The Lewis dot structure of cyclopentane is shown below.

Lewis Dot structure of Cyclopentane

Read More: Unsaturated Solution

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Solved Examples based on Lewis Dot Structures

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1. Define the electron structure of the carbonate ion CO₃^2- in terms of electron dot formulas.

Solution. One potential electron dot formula for the carbonate ions is:

Electron Structure of Carbonate ion

As all the carbon-oxygen bonds are almost equal, over the region of all three carbon-oxygen bonds, one electron pair is delocalized. 

2. What will be the formula and electron dot structure of cyclopentane? 

Solution. In cyclopentane, single bonds connect five carbon atoms of cyclopentane in a cyclic form whose molecular formula is C₅H₁₀. The electron dot structure and the structural formula of cyclopentane are shown below:

Electron Structure of Cyclopentane

Read More: Difference between Cations and Anions

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Things to Remember

• Lewis Dot Structures is covered in Chapter 4 i.e, Chemical Bonding and Molecular Structure of NCERT Class 11 Chemistry.
• The chapter will carry a weightage of 14 periods and 4-5 marks in the exam.
• When it comes to reality, there are many exceptions to their structure. In general, atoms try to fill half or full of their valence electron shell.
• The atoms can do all this and also form molecules that are not stable. 
• In some cases, more than other atoms can also be formed and connect to the central atom.
• The number of valence electrons can sometimes be higher than 8 electrons. Usually, it’s the higher atomic numbers that are observed in this case.
• Lewis structures are helpful when we talk about lighter elements and rather unhelpful when we talk about transition metals such as actinides and lanthanides.