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Lewis Dot Structure

 

In chemistry, the complicated structures of the compounds can be really difficult to understand. So, scientists have developed various methods to depict the atoms, bonds and conformation in a compound in an elaborate yet simple fashion. The methods of representation are:

  1. Lewis Dot Structure: A 2D representation of a compound comprising dots for electrons and lines for bonds.
  2. Bond Line Notation: Primarily used in organic chemistry to represent carbon-containing compounds with lines. Hydrogen atoms are not considered in this representation
  3. Ball and Stick Model: As the name implies, here, the coloured balls correspond to atoms, and sticks correspond to bonds. An efficient way to study the conformation of a compound.
  4. Space Fill Model: The 3D extension of the ball and stick model, where the actual size of an atom is depicted.

Of all the ways mentioned above for depicting a chemical compound, the one we will be focusing on in this article is Lewis Dot Structure.

Introduction:

Lewis dot structure was developed by the scientist Gilbert N Lewis in 1916. This structure has been extensively used to convey information regarding valence electrons and the number of bonds present in an atom.

Valence electrons: The number of electrons present in the outermost shell of an atom are called valence electrons. Every atom wishes to achieve stability by filling its outermost shell with electrons.

Bond: The chemical bonds formed between the atoms are based on the number of valence electrons donated, received or shared between them. If an electron is shared between two atoms, the bond formed is a covalent bond. Also, depending on the number of electrons shared, the number of bonds formed can be single or multiple.

Components of a Lewis Dot Diagram

  1. The chemical symbols of all the atoms participating in the formation of the molecule.
  2. Dots: Represent the number of valence shell electrons of the atom.
  3. Line: bond formed between the two atoms by sharing, donating or receiving electrons.
  4. Charge of the atom: This charge will be determined by the number of lone pairs of electrons present in an atom.

Steps to Draw a Lewis Dot Structure:

A molecule can be graphically represented by following the steps given below:

Step 1: With the help of the electronic configuration of an atom, determine the number of electrons present in the valence shell of the atom.

Step 2: Based on the number of valence shell electrons, calculate the number of electrons needed by the atom to achieve stability. All the elements (exception- Hydrogen and Helium) belonging to the first four rows of the periodic table abide by the octet rule and need eight electrons in the outermost orbital to achieve stability.

Step 3: Subtract the number obtained in step 2 with the number obtained in step 1; this will help us calculate the number of bonds involved in the formation of the molecule.

For the formation of a single bond, two electrons are needed, one from each of the contributing atoms. So, the number of bonds will be half of the number obtained in step 3.

Step 4: In this step, the central atom needs to be decided. This atom generally is the least electronegative compared to the rest. Exceptions: Halogen and Hydrogen, irrespective of their electronegativity, tend to be on the outer side of the molecule and rarely are at the center.

Step 5: Draw a rough skeleton consisting of the central atom, the remaining atoms and the bonds between them.

Step 6: After drawing the skeleton, add the remaining electrons which are not participating in the bond formation around the chemical symbol of the molecule.

Step 7: Compare the structure with the characteristics of the molecule to verify the number of bonds and charges of the molecule. The entire process is based on the trial and error method, so sometimes the structure obtained at the end might be faulty and need rectification.

Lewis Dot Structure of Water

Water: H2O

Electronic Configuration of Oxygen: 1s2 2s2 2p4   Hydrogen: 1s1

Number of Valence Electrons: 6           Number of valence electrons: 1

Number of electrons needed to achieve octet: 2     Number of electrons needed: 1

Number of bonds: (6-2)/2= 2            Number of bonds formed: 1

*Hydrogen is a special molecule and needs to form a duet for stability, hence can contribute in the formation of one bond*

Similarly, Lewis dot structures can be drawn for almost all the compounds; the only requirement is to have a thorough knowledge of the element.

 

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