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Lewis structure (also referred to as Lewis dot diagrams, Lewis dot formulas or Lewis dot structures) are visual representations or diagrams in Chemistry that are used to show the bonding between atoms of a molecule and the lone pair electrons that may exist in the molecule.

A Lewis diagram is thus a simple depiction of the configuration of atoms within a given molecule. It shows each atom and its position in the structure of the molecule using chemical symbols. Electrons are represented as a pair of dots and are placed next to the atom. Note that only valence electrons are represented in the Lewis structure. Lines are drawn between the atoms. These lines show how the atoms are bonded to one another. An atom is considered to be ‘happy’ if the outer electron shell is filled.

Octet rule
The octet rule states that Elements gain, lose or share electrons to fill their outer electron shell with eight electrons.

How to draw a Lewis diagram

You can draw a Lewis diagram for a molecule using the following simple steps.

  • First find the total number of valence electrons. The total number of electrons represented in the Lewis structure is equal to the sum of the number of valence electrons of each individual atom. This gives you the total number of electrons you have.
  • Find the number of electrons required to make a complete octet of electrons in the outer shell. This give you the number of electrons you need.
  • Determine the number of bonds in a molecule. Note that covalent bonds are formed when one electron from each atom forms a pair. Each bond requires two electrons so naturally the number of bonds will be equal to half the number of electrons needed.
  • In order to determine the number of electrons you need to complete the octet, subtract the number of electrons you have from the number of electrons you need.
  • Now draw the central atom and the structure. Connect two atoms by drawing the bonds as determined above. Note that the central atom can connect to four other atoms.
  • Complete the octet for the atoms by adding the number of electrons needed. Create double bonds with the lone pairs of the atom.

Lewis structure examples
Here are some Lewis structure examples

Example 1: NO2-

Lewis Structure | Image source: wikipedia.org

There are five valence electron in nitrogen and six valence electrons in each of the oxygen atoms. So there are a total 17 electrons. The ions have a negative charge so the total number of electrons is 18.

Each oxygen atom must be bonded to the nitrogen atom. It uses four electrons two in each bond. The remaining 14 electrons must be placed in lone pairs. Each oxygen needs three lone pairs to complete the octet. The seventh lone electron must be placed on the nitrogen atom.
The nitrogen atom has only six electrons assigned to it. One of the lone pairs of the oxygen atom must form a double bond.

Each structure has one of the two oxygen atoms double bonded to the nitrogen atom. The second oxygen atom in each structure will be single bonded to the nitrogen atom.

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