📘 IB Definition – Memorise Verbatim
"A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei."
Mark-scoring checklist: Examiners award marks for three components: (1) electrostatic attraction, (2) shared pair of electrons, (3) positively charged nuclei. Missing any one of these loses marks.
Covalent bonding typically occurs between non-metal atoms. Each atom contributes one electron to form a shared pair, stabilising both atoms simultaneously. Most atoms aim for 8 electrons in their valence shell (octet rule), except hydrogen which needs only 2 (duet rule).
Lewis Structure Algorithm
- Count total valence electrons – sum from group numbers. For anions add, for cations subtract.
- Choose the central atom – least electronegative (never H or a halogen).
- Draw single bonds from central atom to each terminal atom (each uses 2 e⁻).
- Complete terminal octets – distribute remaining electrons as lone pairs on outer atoms first.
- Assign remaining electrons to the central atom as lone pairs.
- Check central atom – if it lacks an octet, convert lone pairs from terminal atoms into double/triple bonds.
📐 Worked Example – Methanal (CH₂O)
Step 1: C = 4, H×2 = 2, O = 6 → 12 valence e⁻
Step 2: Central atom = Carbon (least electronegative non-H)
Step 3: 3 single bonds (C–H, C–H, C–O) → 6 e⁻ used, 6 remaining
Step 4: Place 3 lone pairs on O → all 12 e⁻ used
Step 5: Carbon only has 6 e⁻ – needs 2 more
Step 6: Convert 1 lone pair on O → C=O double bond ✓
Result: H₂C=O with 2 lone pairs remaining on oxygen.
Exceptions to the Octet Rule
| Exception | Example | Explanation |
|---|---|---|
| Electron deficient | BF₃ (B has 6 e⁻) | Boron has only 3 valence electrons – stable with an incomplete octet |
| Odd electron | NO (11 e⁻ total) | Cannot pair all electrons – free radical |
| Expanded octet | SF₆ (S has 12 e⁻) | Period 3+ atoms use d-orbitals to exceed 8 electrons |
⚠️ Examiner Trap – Dot-and-Cross Convention
The IB expects Lewis structures to use dots for one atom's electrons and crosses for the other. This shows clearly which atom contributed each electron. Marks are lost for using only dots or only crosses.