Lewis Diagram


Lewis Diagrams — Study Notes

1. What is a Lewis Diagram?

A Lewis diagram (also called a Lewis dot structure) shows the valence electrons of an atom or molecule.

  • Valence electrons = electrons in the outermost energy level.

  • Lewis diagrams use dots to represent valence electrons.

  • For molecules, lines can represent pairs of shared electrons.

Example:

Carbon (C) has 4 valence electrons:

  •
• C •
  •

2. What are Valence Electrons?

Valence electrons are important because they determine how an atom bonds with other atoms.

For the main-group elements, the periodic table can help you find the number of valence electrons:

Group

Valence Electrons

1

1

2

2

13

3

14

4

15

5

16

6

17

7

18

8

Exception: Helium has only 2 valence electrons.

3. The Octet Rule

The octet rule says that atoms generally become more stable when they have 8 valence electrons.

Atoms can achieve this by:

  • Gaining electrons

  • Losing electrons

  • Sharing electrons

Important:

Hydrogen is different because it only needs 2 electrons to be stable.

This is called the duet rule.

4. Lewis Diagrams for Atoms

Steps:

  1. Write the element's symbol.

  2. Find its number of valence electrons.

  3. Place the electrons around the symbol.

  4. Put one electron on each side before pairing them.

Example: Oxygen

Oxygen is in Group 16 → 6 valence electrons.

  ••
• O •
  ••

Oxygen has 2 unpaired electrons.

5. Lewis Diagrams for Ions

An ion is an atom that has gained or lost electrons.

Cations (+)

A positive ion has lost electrons.

Example:

Na → Na⁺

Sodium has 1 valence electron and loses it:

Na⁺

Anions (-)

A negative ion has gained electrons.

Example:

Cl → Cl⁻

Chlorine gains 1 electron and gets a full octet:

  ••
••Cl••
  ••

When drawing an ion, put the Lewis diagram in brackets and write the charge outside.

6. Lewis Structures for Covalent Compounds

A covalent bond happens when atoms share electrons.

Usually, covalent bonds occur between non-metals.

One shared pair = one single bond

A line represents 2 shared electrons.

H—H

The two hydrogen atoms share 2 electrons.

Each hydrogen now has 2 electrons → stable.

7. How to Draw Lewis Structures

Step 1: Count the total valence electrons

Add the valence electrons from every atom.

Step 2: Choose the central atom

Usually:

  • The atom that appears only once is the central atom.

  • Hydrogen is NEVER the central atom.

  • Carbon is often the central atom.

Step 3: Connect the atoms with single bonds

Each single bond uses 2 electrons.

Step 4: Complete the outer atoms' octets

Give the outside atoms enough electrons to reach 8.

Remember: Hydrogen only needs 2.

Step 5: Put leftover electrons on the central atom

Try to give the central atom 8 electrons.

Step 6: Check your work

Count all the electrons again.

The number of electrons in your diagram should equal the total number of valence electrons you calculated.

8. Example: H₂O

Water = H₂O

Step 1: Count valence electrons

H = 1 each

O = 6

2(1) + 6 = 8 electrons

Step 2: Choose the central atom

Oxygen goes in the middle:

H O H

Step 3: Add single bonds

H—O—H

Two bonds = 4 electrons.

Step 4: Add remaining electrons

There are 4 electrons left, so put them on oxygen:

  ••
H—O—H
  ••

Oxygen now has 8 electrons around it.

9. Example: CO₂

Carbon dioxide = CO₂

Step 1: Count valence electrons

C = 4

O = 6 × 2 = 12

Total = 16 electrons

Step 2: Choose the central atom

Carbon goes in the middle:

O—C—O

Step 3: Complete the outer atoms

After giving each oxygen 8 electrons, carbon does not have 8.

So we need to make double bonds.

Final structure:

O=C=O

Each double bond represents 4 shared electrons.

10. Single, Double, and Triple Bonds

Single bond

1 shared pair = 2 electrons

A—B

Double bond

2 shared pairs = 4 electrons

A=B

Triple bond

3 shared pairs = 6 electrons

A≡B

11. Lone Pairs

A lone pair is a pair of valence electrons that is not involved in bonding.

Example:

  ••
H—O—H
  ••

The two pairs of dots on oxygen are lone pairs.

12. Quick Rules to Remember

⭐ H → wants 2 electrons

⭐ C → usually wants 4 bonds

⭐ N → usually wants 3 bonds + 1 lone pair

⭐ O → usually wants 2 bonds + 2 lone pairs

⭐ F, Cl, Br, I → usually want 1 bond + 3 lone pairs

⭐ One line = 2 electrons

⭐ Two lines = 4 electrons

⭐ Three lines = 6 electrons

⭐ Always count the total valence electrons first!

⭐ Hydrogen is never the central atom.

⭐ Check that your final structure uses the correct number of electrons.

13. Common Mistakes

❌ Forgetting to count all valence electrons.

❌ Giving hydrogen 8 electrons.

❌ Making hydrogen the central atom.

❌ Forgetting lone pairs.

❌ Using too many or too few electrons.

❌ Forgetting that a line represents 2 electrons.

❌ Forgetting the charge when drawing an ion.

14. Lewis Diagram Checklist

Before you finish, ask:

☐ Did I count all the valence electrons?

☐ Did I choose the correct central atom?

☐ Did I connect the atoms?

☐ Did I complete the outer atoms' octets?

☐ Does hydrogen have 2 electrons?

☐ Does the central atom have 8 electrons when possible?

☐ Did I use the correct total number of electrons?

☐ Did I include the charge for an ion?

🧠 Super Short Version

Lewis diagrams show valence electrons.

Steps:

1. Count valence electrons → 2. Choose central atom → 3. Connect atoms → 4. Complete outer octets → 5. Put remaining electrons on central atom → 6. Check total electrons.

Octet rule = atoms generally want 8 valence electrons.

Hydrogen = 2 electrons.

1 line = 2 shared electrons.

2 lines = 4 shared electrons.

3 lines = 6 shared electrons.