Lewis Diagrams (College Board AP Chemistry)

• A Lewis Diagram is used to represent molecular compounds by showing the valence electrons around atoms
• Electrons are drawn in pairs and they are represented by For example, the water molecule can be represented as:

Lewis Diagrams for Water

Different Lewis Diagrams for the water molecule showing bonding pairs as lines or dots

• Lewis diagrams must show all the electron pairs even if they are bonding or nonbonding electrons
  • A bonding pair is a shared pair of electrons
  • A nonbonding pair is a lone pair of electrons that is not involved in bonding
• Double bonds are represented with two lines connecting the two atoms involved (＝)
  • This implies that 4 electrons are being shared between them
• Triple bonds are represented with three lines connecting the two atoms involved ( ≡ )
  • This implies that 6 electrons are being shared between them

• Atoms form covalent compounds to have a full valence shell of electrons
• The octet rule is the tendency that explain why atoms are most stable when they complete their valence shell with 8 electrons
• However, there are some exceptions to the octet rule:
  • The hydrogen atom (H) completes a This means it can only complete its valence shell with 2 electrons
  • The boron atom (B) completes a This means it can only complete its valence shell with 6 electrons

Steps for drawing Lewis Diagrams

1. Calculate the total number of valence electrons using your periodic table
2. Draw a correct skeletal structure to show how the atoms are linked between them
3. Distribute one pair of electrons between each atom to represent single bonds
4. Add electron lone pairs to complete octets (except H which completes two valence electrons). It is a good practice to start on terminal atoms and finish with the central atom
5. If there are still atoms which lack an octet, form double or triple bonds
6. When the structure is finished, check if all the atoms have completed an octet (except from H) and check if the amount of electrons distributed is the same as the total number of valence electrons

Further examples of drawing Lewis Diagrams

• By following six simple steps, the Lewis Diagrams of these common molecules can be drawn

Molecule	Total number of valence electrons	Lewis Diagram
CH4	C + 4H 4 + ( 4 x 1) = 8
NH3	N + 3H  5 +( 3 x 1)=8
H2O	2H + O (2 x 1) + 6 =8
CO2	C + 2O 4 + ( 2 x 6) = 16
HCN	H+C+N 1+ 4 + 5 = 10

Lewis diagrams for common molecules