Lewis Dot Structures

Lewis Dot Structures - Definition and Example

Lewis structure is basically a graphic representation of the electron distribution around an atom. The major reason why learning Lewis dot structure is important is that it helps in predicting the number and type of bonds which can be formed around an atom. It also helps in predicting the geometry of the molecule. Learning to make proper Lewis dot structures can help in solving the problem that most of the students have.

Below are the steps to find the lewis dot structure

How to draw Lewis structure

Drawing the Lewis structure is very important as it is then only when one can calculate the formal charge correctly. Thus below are the steps to draw the Lewis structure.
We shall take an example of NO3-.

Step 1 

Determine the number of valence electrons present in the molecule.

Nitrogen 5
Oxygen3 x 6 = 18
negative charge1

Thus, the total number of valence electrons is 24.

Step 2

Then draw a skeleton molecule in which the central atom connects all the atoms using a single bond. In this case, nitrogen is the central atom and connects three oxygen using three single bonds.

Step 3 

Out of the 24 electrons, 6 electrons have been used to make the skeleton. Now use the 18 electrons to fill the octet. Fill the most electronegative atom first and then the electropositive atom.

Step 4 

Now check if the octets are filled or not, if they are filled then make multiple bonds.

Step 5

Now check which has the lowest formal charge. The one with the lowest formal charge is the most stable one.

Formal charge formula 

the formal charge can be expressed mathematically using the following formula,

Formal charge = ( Total number of valence electrons in a free state) - (Total number of electrons assigned in Lewis structure)
Formal charge = (Total number of valence electron in a free state) - ( Total number of nonbonding pair of electrons [lone pair]) - ½ (total number of bonding electrons)

Factor ½ is attached to a number of bonding electrons because the bonding electrons are shared between two atoms.

Let us now calculate the formal charge for SO4-2

The Lewis structure of SO4-2 is as follows,

The oxygen in red color is O- 1
The oxygen in Green color is O-2
The oxygen in blue color is O-3
The oxygen in pink color is O-4

AtomValence e– in free stateNo. of nonbonding e– in Lewis structureNo. of bonding pairs in Lewis structureFormal Charge
Sulphur - S6012= 6 – 0 – 12/2=6-6=0
Oxygen -O- 1644= 6 - 4 - 4/2= 2-2= 0
Oxygen -O- 2662= 6 - 6 - 2/2= 0 -1= -1
Oxygen -O- 3644= 6 - 4 - 4/2= 2 - 2 = 0
Oxygen -O- 4662= 6 - 6 - 2/2= 2 - 1= -1

Importance of formal charge 

  • • The formal charge is a theoretical charge and thus does not indicate any real charge separation present in the molecule.

  • • They help in selecting the lowest energy structure from all the possible Lewis structure of a given molecule

  • • Having knowledge about the lowest energy structure can help in predicting the major product of a given reaction and also helps in understanding the phenomenon a lot.

  • • In general, the lowest energy structure has the smallest formal charge on the atom and also has the most distributed charges too.

  • Real molecules Vs Lewis structures

    Lewis structures are really helpful when it comes to learning about the oxidation states, valence and the type of bonding. But in reality, there are many exceptions when it comes to the structure in reality. Atoms in general, try and seek to half- fill or fully-fill their valence electron shell. But, the atoms can do this and also form molecules which are not stable. In some cases, the central atom can also form more than the other atoms connected to it. There are chances that the number of valence electrons can exceed more than 8 electrons. This can mostly be seen in higher atomic numbers. Lewis structures are generally helpful when it comes to lighter elements and not that helpful when it comes to transition metals including both actinides and lanthanides. They depict the irregular arrangement of electrons around the atom.