IB Chemistry›Structure 2›2.2›2.2.1
2.2.1

Covalent Bond & Lewis Formulas

Electrostatic attraction and the octet rule.

The Covalent Bond

Definition

The electrostatic attraction between a shared pair of electrons and the positively charged nuclei.

The Octet Rule

Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons (stable noble gas configuration).

Exceptions
  • Hydrogen (H): Stable with 2 electrons.
  • Beryllium (Be): Stable with 4 electrons (e.g., BeCl₂).
  • Boron (B): Stable with 6 electrons (e.g., BF₃).

Drawing Lewis Formulas

  1. Count Valence e⁻: Sum group numbers. Adjust for ions.
  2. Skeleton: Connect atoms with single bonds (least electronegative in center).
  3. Fill Outer: Complete octets for outer atoms (duet for H).
  4. Check Center: If center lacks octet, share lone pairs to form double/triple bonds.

Worked Example: Lewis Formula for CO₂

Problem: Draw the Lewis structure for carbon dioxide.

1. Count valence e⁻: C = 4, O = 6 × 2 = 12. Total = 16 e⁻.

2. Skeleton: O – C – O (C is least electronegative → central).

3. Fill outer atoms: Each O needs 8 e⁻. Using single bonds only: 2 (bonds) + 12 (lone pairs) = 14. That leaves 2 spare — but C only has 4 e⁻ around it.

4. Form double bonds: Share two lone pairs from each O → O=C=O. Now C has 8 e⁻ and each O has 8 e⁻.

Answer: O=C=O — two double bonds, two lone pairs on each oxygen.

Think About It

Why does carbon almost always form 4 bonds, never 3 or 5?

Carbon has 4 valence electrons and needs 4 more to complete its octet. It cannot expand its octet because it has no accessible d-orbitals (Period 2 element). This makes it uniquely versatile — it can form 4 single bonds, 2 doubles, 1 double + 2 singles, or 1 triple + 1 single.

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