Introduction
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.
There are the following topics covered in this article:
What are Lewis Dot Structures?
How to draw Lewis Structure: Steps with Example
Formal Charge and its Mathematical formula
Calculation of the Formal Charge for SO4-2
Importance of Formal Charge
Real Molecules V/s Lewis Structure
FAQs
How to Draw Lewis Structure
Below are the Steps to Find the Lewis Dot 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.
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 electrons in a free state) - ( Total number of nonbonding pairs of electrons lonepair) - 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 at the top is O-1
The oxygen on the left side is O-2
The oxygen on the right side is O-3
The oxygen at the bottom is O-4
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 structures 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 that 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.
FAQs on Lewis Dot Structures
1. What is lewis dot structure in chemistry?
Lewis Dot Structure is mainly a graphic representation of the valence shell electrons of different atoms in Chemistry. It was invented by a renowned physical chemist of America, Gilbert Newton Lewis (1875 - 1946), who had introduced and mentioned this in his 1916 published article under the topic - The Atom and the Molecule. Lewis dot structure is also known by some other names such as electron dot structure, Lewis electron-dot formula, or Lewis Electron Dot Structures (LEDs), etc. It has many applications in Chemistry, especially for the topics related to the study of bonds between atoms, the atomic structure of atoms, molecular & organic Chemistry, etc.
2. Are there any limitations of Lewis Dot Structure?
Yes, there are some limitations or disadvantages of Lewis electron dot structure as well. Lewis electron dot structures are based on the octet rule of atoms, they explain and let us predict the type and number of bonds formed by elements, based on the number of electrons of the outermost cells. But, fails to explain the geometry of molecules, is limited to the 2-dimensional representation of molecules through diagrams only, does not accurately describe the magnetic behaviour of molecules and bonds, etc.
3. What are the applications of Lewis Dot or Electron Dot structure?
There are many applications of Lewis Dot Structures. They are used to study and understand chemical bonds, atomic structures i.e. type of bonds formed by atoms using valence electrons or/and lone pairs, and the number of bonds an atom can or will make with other atoms. Lewis electrons dot diagrams are also useful in predictive analytics. They help scientists and students to predict the bond strength, bond length, shape of molecules, their oxidation states, etc. Lewis electron diagrams make it overall easier for the students to study and know more about chemical bonds, formation, their reactivity with other atoms, and the geometry of molecules before and after forming bonds.
4. How to draw Lewis a Dot Structure of molecule? Explain with the steps involved and an example.
Lewis dot structure is the representation of electrons of atoms, with the help of a diagram. The diagram consists of valence electrons or electrons from the outermost shell of atoms in the form of Dots, so as to make Lewis Electron Dot Structure of atoms. Below are some steps that the students may consider and follow, in order to make or draw Lewis Dot Structure, these steps are:
Firstly, let us take a molecule, say - NO3- for which we will learn to draw the Lewis Dot Structure. (Please note: NO3- is not a neutral molecule, but an ionic molecule, with a negative charge, = a.)
Now, we will write down the number of electrons present in each atom of the molecule (NO3-). Like, N = 5, O = 6*3 = 18. Plus, the ionic charge present in NO3-, which is = 1.
Students will now sum up the total number of valence electrons of the molecule, which is = 5+18+1 = 24. (step - b).
Start drawing a diagram, keeping that atom in the centre, which connects other atoms of the molecule, like N, in case of NO3- as N (Nitrogen) makes 3 bonds with O (Oxygen) in NO3-.
Following the octet rule of Chemistry, students will represent electrons as dots around each atom of the molecule. For example: in the case of NO3-, students will draw 6 dots (3 lone pairs) around each of three O atoms, keeping the N in the centre, which also makes a covalent bond with each of three O atoms, in NO3-.
Finally, once the structure is drawn, it is to ensure that the Formal Charge of the molecule is lowest, as it is inversely proportional to the stability of the molecule. The lowest the formal charge, the more will the stability of the molecule.
5. What is Formal Charge, and how is it related to Lewis electron dot structure?
Formal Charge of a molecule is associated with its stability. It basically represents the overall charge on each atom of a molecule. The mathematical equation to calculate the formal charge of a molecule is F = V - L - B/2. Where F represents Formal Charge, V denotes the number of valence electrons of the molecule, L is the number of valence electrons not forming bonds, and B total number of shared electrons in the bond formation of the molecule. This is the most popular and used a formula to calculate formal charges on molecules. Formal Charge is related to Lewis Dot Structure as it calculates the overall charge of a molecule with the lowest energy structure possible. Which helps in predicting the stability of a molecule with Lewis Dot Structure.
6. Can we find through Lewis Dot Structure that how many bonds will a molecule form?
Yes, it is quite possible to determine or predict the number of bonds a molecule will form based on the Lewis Dot Structure. While making an electron dot structure of molecules, it is required to find out and represent the total formal charge of the molecule too, which not only helps predict the stability of the molecule but also the type and number of bonds a molecule will form. With Lewis dot structures, it is easier to represent electrons as dots and analyze the most stable structure a molecule can make, just by following the octet rule and common formula of calculating the formal charge of the molecules.
7. Is there any purpose of drawing Lewis Dot Structure?
Definitely, the main objective of Lewis Dot Structure is to support students and scientists with their predictions related to the most stable structure that a molecule can have while making bonds with other molecules. There are plenty of other applications and advantages of drawing molecular structures with the Lewis Dot technique. Based on the simple graphical representations of valence electrons present in the outermost shell of atoms, it helps in predicting the type and number of bonds, overall formal charge, stability, chemical bond formation, and properties of molecules.
