Writing Lewis Structures

The Lewis structure is named after Gilbert N. Lewis, who developed the concept in 1916. The idea was later introduced and named by Irving Langmuir in 1920, who gave credit to Lewis. Lewis structures are also known as electron dot structures. Lewis introduced simple notations which represent bond between atoms and valence electron represented by dots on atoms. These notations are called Lewis symbols or Lewis dot structure.

Lewis symbol for second period elements are 

 Lewis symbol for some anions are 

How to draw Lewis Diagrams?

The nitrogen atom has five valence electrons, and we know from the chart that it needs three more electrons to achieve a full octet. The oxygen atom has six valence electrons, and we know that it requires two more electrons to complete a full octet. We can see from the chart that nitrogen will give up three valence electrons, and oxygen will gain two valence electrons by forming a triple bond between the two atoms.

Let us draw a Lewis structure for NO3-. First, we need to calculate the number of valence electrons in a molecule. It can be calculated by adding the group number of all the atoms and their charge. After that, we would have obtained the number of valence electrons.

N(Valence shell electron= 5)

O (*3) 18

Charge 1

Total: 24

The octet rule requires that the central atom have eight electrons in its outer shell. If you run out of electrons before forming bonds, you have to create multiple bonds to get it done. The number of multiple bonds varies on what type of element you’re dealing with. Here, we can see that N is short with electrons. Now, we can use one lone pair from the atom of O, located on the left. Hence, it will form a double bond with one of the oxygen and help complete the bond.

Now, we need to determine the formal charge of the molecule. The formal charge of each atom in a molecule is significant because it indicates the polarity of the molecule and the direction of bonding. All atoms with a positive formal charge are electrophiles and are usually the attacking species in electrophilic addition reactions. Atoms with a negative formal charge (indicated by a minus sign) are nucleophiles and are traditionally the attacking species in nucleophilic substitution reactions.

We cannot complete the Lewis structure without the formal charges. The things which have been needed in this are as follows:

• It is necessary to have the formal charges the same as the atom’s electronegativity. It means more negative charges on the electronegative atoms and less positive charges on the electronegative atoms.

The formal charge is the charge it would have if all its bonds were utterly ionic. If you have a -1 and +1 charge close together, you can often rearrange things so that the charges cancel out. One way to do this is to use the lone pair of electrons on the -1 atom to form a double bond with the nearest +1 atom. This makes a double bond between them instead of a single bond, and there is no longer a net charge.

The Lewis structure for NO3- :

You have determined that the “best” structure involves a double bond between the nitrogen and one of the oxygens (shown in red). That is fine, but there are several ways to display this information. 

Postulates Of Kossel-Lewis Theory

Kossel has discussed several facts about chemical bonding, and the important postulates comprise the following:

1. The highly electronegative halogens, along with highly electropositive alkali metals, seem to be segregated by the noble gases available under the periodic table.

2. The creation of the negative ion from halogen and positive ions from the alkali metals are related to electron gain and loss.

3. The positive ions and negative ions make the stable electronic configuration of noble gas configuration.

4. Each noble gas of the periodic table has the stable electronic configuration of the octet, excluding Helium which comprises duplet configuration.

Example: considering the formation of NaCl, NaNaCl, Na loses an electron, which is gained by chlorine. By losing an electron, NaNa will achieve the stable configuration of Neon, while ClCl, on gaining an electron, will attain the stable configuration of Argon.

Na      →           Na++1e–

[Ne]3s1                  [Ne]

Cl+1e–         →          Cl1-

[Ne]3s23p5                    [Ar]

Conclusion :

The number of dots surrounding the symbol will show the number of available valence electrons, which helps calculate the standard or group valence of the particular element.

According to the concept of the Lewis Structures, the atom related to different elements took active participation in combination to complete their octet (8 electrons in the outermost valence shell) or duplet ( 2 valence electrons in the outer valence shell )for attaining the nearest noble gas configuration. This complete process is known as OCTET RULE.