Ionic Bonding

• An ionic compound is electrically neutral overall. The total positive charge from the cations exactly cancels the total negative charge from the anions
• Two equivalent methods for working out the formula: direct comparison and swap-and-drop

Direct comparison

• Pick the smallest whole-number ratio of cations to anions that makes the total charge balance to zero
• Iron(II) sulfate: Fe²⁺ has +2, SO₄²⁻ has −2; (+2) + (−2) = 0, so 1 Fe²⁺ pairs with 1 SO₄²⁻
• Formula: FeSO₄

Swap-and-drop

• Faster when the cation and anion have different charge sizes
• Procedure:
  1. Write the two ions side by side, with their charge numbers as superscripts
  2. Swap each ion's charge size to become the subscript of the other ion
  3. Drop the +/− signs; drop any subscript of 1 (it is always implied)
  4. Simplify the subscripts if they share a common factor
• Examples:
  • Copper(II) chloride: Cu²⁺ and Cl⁻ → swap gives Cu₁Cl₂ → drop the 1 → CuCl₂
  • Iron(III) chloride: Fe³⁺ and Cl⁻ → swap gives Fe₁Cl₃ → FeCl₃
  • Aluminium oxide: Al³⁺ and O²⁻ → swap gives Al₂O₃ (no common factor)
  • Calcium sulfide: Ca²⁺ and S²⁻ → swap gives Ca₂S₂ → simplify by 2 → CaS

Brackets for polyatomic ions

• Whenever a polyatomic ion is multiplied (subscript ≥ 2), put the polyatomic ion in brackets before applying the subscript:
  • Magnesium hydroxide: Mg²⁺ and OH⁻ → Mg(OH)₂
  • Aluminium nitrate: Al³⁺ and NO₃⁻ → Al(NO₃)₃
  • Ammonium sulfate: NH₄⁺ and SO₄²⁻ → (NH₄)₂SO₄

Naming ionic compounds

• The metal (or the ammonium ion) keeps its name in full
• The non-metal usually changes its ending to -ide:
  • chlorine → chloride
  • oxygen → oxide
  • sulfur → sulfide
  • nitrogen → nitride
• For metals that can form more than one cation (Fe, Cu, Pb), include the charge in Roman numerals: iron(II), iron(III), copper(II), lead(II)
• Polyatomic anions keep their proper name: nitrate, carbonate, sulfate, hydroxide