The Ionic Bond
"The ionic bond is formed by electrostatic attractions between oppositely charged ions."
Key Characteristics
Non-Directional Nature
Unlike covalent bonds, which are highly directional (along an axis between two atoms), the ionic bond is strictly non-directional. The electrostatic field radiates isotropically from a charged ion. A single Na⁺ does not "belong to" a single Cl⁻. Instead, it attracts every adjacent Cl⁻ in its vicinity while simultaneously repelling other Na⁺ ions.
This continuous, multi-directional attraction is the genesis of the macroscopic, repeating crystal lattice.
Reactivity 3.2 (Redox)
The formation of an ionic compound from its elements is fundamentally a redox reaction. Electron transfer is a dual process:
Oxidation: Na → Na⁺ + e⁻ (loss of electrons)
Reduction: ½Cl₂ + e⁻ → Cl⁻ (gain of electrons)
Overall: 2Na(s) + Cl₂(g) → 2NaCl(s)
IUPAC Nomenclature
For binary ionic compounds, the cation (metal) name comes first, followed by the anion (non-metal) name with its suffix changed to "-ide".
| Ions | Formula | Name |
|---|---|---|
| Mg²⁺ + O²⁻ | MgO | Magnesium oxide |
| Ca²⁺ + P³⁻ | Ca₃P₂ | Calcium phosphide |
| Fe²⁺ + Cl⁻ | FeCl₂ | Iron(II) chloride |
| Fe³⁺ + Cl⁻ | FeCl₃ | Iron(III) chloride |
Transition metals with variable charges use Stock notation: a Roman numeral in brackets after the metal name to indicate the specific oxidation state.
Deducing Empirical Formulas
The total crystalline structure must remain electrically neutral. The sum of positive charges must exactly cancel the sum of negative charges.
Aluminium Oxide Formula Deduction
Step 1: Identify ions: Al³⁺ and O²⁻
Step 2: Find the LCM of charges 3 and 2 → 6
Step 3: 2 × Al³⁺ = +6 and 3 × O²⁻ = −6 → balanced
Al₂O₃
Polyatomic Ions. Must Memorise
A polyatomic ion is a tightly bound group of covalently bonded atoms that behaves as a single unit carrying a net charge. These are NOT in the Data Booklet.
| Name | Formula | Charge |
|---|---|---|
| Ammonium | \(\text{NH}_4^+\) | 1+ |
| Hydroxide | \(\text{OH}^-\) | 1− |
| Nitrate | \(\text{NO}_3^-\) | 1− |
| Hydrogencarbonate | \(\text{HCO}_3^-\) | 1− |
| Carbonate | \(\text{CO}_3^{2-}\) | 2− |
| Sulfate | \(\text{SO}_4^{2-}\) | 2− |
| Phosphate | \(\text{PO}_4^{3-}\) | 3− |
Brackets for Polyatomic Ions
When more than one polyatomic ion is required, brackets are mandatory.
✓ (NH₄)₂SO₄ ✗ NH₄₂SO₄
Omitting brackets is a "fatal error" and results in zero marks for formula deduction on mark schemes.
Defining the Ionic Bond
Students frequently define an ionic bond as "the transfer of electrons". This describes the formation of the ions, not the bond itself. The correct definition is: "the electrostatic attraction between oppositely charged ions". Examiners consistently penalise this error.
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