Ions & Ionic Bonds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
Forming Ions
Atoms can gain or lose electrons to become charged particles called ions, achieving a stable, full outer shell like the nearest noble gas.
- Cation (positive ion): formed when an atom loses electrons - mainly formed by metals (Groups I-III)
- Na (2.8.1) loses 1 electron → Na⁺ (2.8)
- Mg (2.8.2) loses 2 electrons → Mg²⁺ (2.8)
- Anion (negative ion): formed when an atom gains electrons - mainly formed by non-metals (Groups V-VII)
- Cl (2.8.7) gains 1 electron → Cl⁻ (2.8.8)
- O (2.6) gains 2 electrons → O²⁻ (2.8)
For main groups, the charge can be predicted from the group number: Group I → 1+, Group II → 2+, Group III → 3+, Group V → 3-, Group VI → 2-, Group VII → 1-.
Ionic Bond Formation
An ionic bond is a strong electrostatic force of attraction between oppositely charged ions. It forms between a metal and a non-metal, when electrons are transferred (not shared) from the metal to the non-metal.
Worked Example: Sodium Chloride
- Question: use electronic configurations to explain the formation of the ionic bond in sodium chloride, NaCl. [3]
- Answer: sodium (2.8.1) transfers its 1 outer electron to chlorine (2.8.7) [1]; sodium becomes Na⁺ (2.8), with a stable configuration like neon [1]; chlorine becomes Cl⁻ (2.8.8), with a stable configuration like argon; the oppositely charged Na⁺ and Cl⁻ ions are then held together by strong electrostatic attraction [1]
In ionic bonding electrons are transferred, not shared - each ion is drawn separately in its own brackets with its charge shown outside
Formulas of Ionic Compounds
Ionic compounds are electrically neutral overall, so the total positive charge must equal the total negative charge. This fixes the ratio of ions in the formula.
| Cation | Anion | Balancing | Formula |
|---|---|---|---|
| Na⁺ | Cl⁻ | 1:1 | NaCl |
| Mg²⁺ | Cl⁻ | 1:2 (two Cl⁻ needed) | MgCl₂ |
| Al³⁺ | O²⁻ | 2:3 (2×3 = 3×2 = 6) | Al₂O₃ |
Common polyatomic ions to know: hydroxide OH⁻, nitrate NO₃⁻, carbonate CO₃²⁻, sulfate SO₄²⁻, and ammonium NH₄⁺ (the only common positive polyatomic ion). Brackets are used when more than one polyatomic ion is needed, e.g. Ca(OH)₂.
Naming: the metal ion keeps its element name; the non-metal ion's ending changes to -ide (chloride, oxide) - except polyatomic ions, which keep their own names (sulfate, carbonate, nitrate).
Common Mistakes
- ❌ Thinking ionic bonding involves sharing electrons, like covalent bonding
- ✓ ionic bonding involves the transfer of electrons from metal to non-metal, forming two separate charged ions - covalent bonding is what involves sharing
Interactive revision notes, videos and practice questions load below.