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2.2.15Additional lessons

Formal Charge (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 6 parts Lesson 39 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 6The Formula

Formal charge compares the electrons an atom owns in a structure with those in the free atom.

Equation
  • Equation Formal charge = V − (L + B/2), where V is the valence electrons of the free atom.
L
  • L The number of non-bonding electrons on the atom in the structure.
B
  • B The number of electrons in bonds around the atom, so B/2 is the number of bonds.
Total
  • Total Formal charges add up to the overall charge of the species.

Section 2 of 6Choosing the Best Structure

1
Draw all possible Lewis structures for the species and work out the formal charge on every atom.
2
The structure with formal charges closest to zero on all atoms is generally preferred.
3
If charges are needed, the negative charge should be on the more electronegative atom.
4
Avoid structures with like charges on adjacent atoms.
5
Remember that formal charges are bookkeeping and are not the real charges on atoms.

Section 3 of 6Worked Cases

Worked Cases
CO₂
  • CO₂ In O=C=O all formal charges are zero, so this structure is better than ones with charges separated.
NO₃⁻
  • NO₃⁻ Nitrogen has +1, the double-bonded oxygen has 0 and each single-bonded oxygen has −1.
SO₄²⁻ single bonds
  • SO₄²⁻ single bonds With four single bonds, S has +2 and each O has −1.
SO₄²⁻ expanded
  • SO₄²⁻ expanded With two S=O bonds, S has 0 and the charge is carried by two oxygens at −1.
Atom in SO₄²⁻All single bondsTwo S=O bonds
S+20
Single-bonded O−1−1
Double-bonded O–0

Section 4 of 6Uses and Limits

Predicting structure
  • Predicting structure It supports drawing expanded octet structures where formal charges are minimised.
Limitation
  • Limitation It ignores electronegativity differences, so the real charge distribution differs.
Resonance
  • Resonance Equivalent structures have the same formal charges and so contribute equally.

Section 5 of 6Worked Example

Question Find the formal charges in NO₃⁻ with one N=O and two N–O⁻ bonds.

  1. N: V = 5, L = 0, B = 8, so FC = 5 − (0 + 4) = +1.
  2. Double-bonded OV = 6, L = 4, B = 4, so FC = 6 − (4 + 2) = 0.
  3. Each single-bonded OV = 6, L = 6, B = 2, so FC = 6 − (6 + 1) = −1.

Answer +1 + 0 + (−1) + (−1) = −1, which matches the ion charge.

Section 6 of 6Exam Checklist & Tips

2.2.15 – Key Points for the Exam

  • Formal charge The charge an atom would have if bonding electrons were shared equally.
  • Valence electrons The outer-shell electrons of the free atom.
  • Non-bonding electrons Electrons in lone pairs on the atom.
  • Preferred structure The Lewis structure with formal charges nearest zero.

Common Mistakes

  • Counting bonding electrons instead of half of them (B/2).
  • Confusing formal charge with oxidation state.
  • Forgetting that the sum must equal the charge on the ion.
  • Treating formal charge as the actual charge on an atom.

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