Ionic Compounds
Summary: Compounds of oppositely charged ions held in a giant lattice, characterised by high melting points and electrical conductivity only when molten or aqueous. Tags: igcse chemistry Created: 2026-07-18
An ionic compound is a Compound formed when metal atoms transfer electrons to non-metal atoms, producing positively charged metal ions (cations) and negatively charged non-metal ions (anions) that are held together by the strong electrostatic attraction of the Ionic Bond. The ions do not exist as discrete pairs or molecules; instead they pack into a Giant Ionic Lattice — a regular, repeating three-dimensional arrangement of alternating positive and negative ions extending throughout the crystal. Typical examples include sodium chloride (NaCl), magnesium oxide (MgO) and other salts formed between metals from Groups 1 and 2 and non-metals from Groups 6 and 7 of the Periodic Table. The formation of the ions themselves can be represented with a Dot-and-Cross Diagram, and the overall formula reflects the ratio of ions needed for the charges to cancel. Because their structure and bonding are so different from the covalently bonded simple molecules described under Covalent Bonding, ionic compounds have a distinctive set of physical properties — notably high melting and boiling points and conductivity that depends on the physical state — which are examined below and exploited in processes such as Electrolysis.
Properties of Ionic Compounds
| Property | Ionic compounds | Explanation |
|---|---|---|
| Melting and boiling points | High (e.g. NaCl melts at 801 °C; MgO at 2852 °C) | Strong electrostatic attractions act in all directions throughout the Giant Ionic Lattice; a large amount of energy is needed to overcome them. |
| Electrical conductivity (solid) | Poor — does not conduct | Ions are fixed in position in the lattice and cannot move to carry charge. |
| Electrical conductivity (molten or aqueous) | Good — conducts | The lattice is broken down, so the ions are free to move and carry charge. |
| Solubility | Many are soluble in water | Water molecules can separate and surround the ions (see Solubility Rules). |
| State at room temperature | Crystalline solids | The regular lattice produces hard, brittle crystals with flat faces. |
High Melting and Boiling Points
The Ionic Bond is a strong electrostatic attraction between oppositely charged ions, and in a Giant Ionic Lattice this attraction acts in all directions between every ion and all of its oppositely charged neighbours. Melting or boiling an ionic compound means overcoming a very large number of these strong attractions throughout the whole structure, which requires a great deal of thermal energy. Ionic compounds are therefore solids at room temperature with high melting and boiling points.
The size of the ionic charges matters: magnesium oxide (Mg²⁺ and O²⁻) has far stronger attractions than sodium chloride (Na⁺ and Cl⁻), so its melting point (2852 °C) is much higher than that of NaCl (801 °C).
Electrical Conductivity
For a substance to conduct electricity, it must contain charged particles that are free to move.
- Solid ionic compounds do not conduct: the ions are held in fixed positions in the lattice by strong electrostatic forces and can only vibrate, so no charge can flow.
- Molten ionic compounds do conduct: melting breaks up the lattice, so the ions become mobile and can move towards the electrodes, carrying charge.
- Aqueous solutions of ionic compounds do conduct: when dissolved in water, the lattice separates into free-moving hydrated ions.
This mobile-ion conduction is the basis of Electrolysis, in which a molten or aqueous ionic compound is decomposed by an electric current: cations migrate to the cathode and anions to the anode. Note that the charge carriers are ions, not the delocalised electrons responsible for conduction in metals (Metallic Bonding).
Brittleness
Although hard, ionic crystals are brittle. If a layer of ions is forced to slide, ions of like charge are brought side by side; the resulting repulsion splits the crystal apart. This contrasts with metals, whose layers can slide without breaking the metallic bond.
Comparison with Simple Covalent Substances
| Feature | Ionic compound | Simple covalent molecule |
|---|---|---|
| Particles | Oppositely charged ions | Neutral molecules |
| Structure | Giant Ionic Lattice | Small molecules with weak intermolecular forces |
| Melting/boiling point | High | Low |
| Conducts when solid | No | No |
| Conducts when molten/aqueous | Yes | No (no ions or free electrons) |
Sources
- Cambridge IGCSE Chemistry 0620 Syllabus (2023-2025) — Section 2.4 (Core 4, Supplement 7), Cambridge Assessment International Education
Common Misconceptions
| Misconception | Reality |
|---|---|
| Ionic compounds are made of molecules of NaCl, MgO, etc. | They consist of a giant lattice of ions; the formula only gives the simplest ratio of ions, not a discrete molecule. |
| Ionic compounds never conduct electricity. | They conduct well when molten or dissolved in water, because the ions are then free to move; only the solid is a non-conductor. |
| Electricity is carried through molten ionic compounds by electrons. | The charge carriers are the mobile ions themselves; free electrons carry charge only in metals and graphite. |
| Melting an ionic compound breaks the ions apart into atoms. | Melting only frees the ions from their fixed lattice positions; the ions themselves remain intact and charged. |
| The high melting point is caused by strong forces between molecules. | There are no molecules; the strong electrostatic attractions act between oppositely charged ions throughout the entire lattice. |