Properties of ionic compounds
The physical propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point. of ionic compoundAn ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). can be explained by thinking about their structure and bonding.
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High melting points and boiling points
Ionic compounds are solids at room temperature. Melting and boiling are stateSolid, liquid or gas. Evaporation is a change of state from liquid to gas. changes.
Energy has to be transferred to a substance in order to melt or boil it. This energy is needed to break the bonds between particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. in the substance:
- some bonds are overcome during melting
- all remaining bonds are overcome during boiling
The more energy needed, the higher the melting pointThe temperature at which a solid changes into a liquid as it is heated. or boiling pointThe temperature at which a substance rapidly changes from a liquid to a gas..
Explanation
Ionic compounds are held together by many strong electrostatic forceA force of attraction between particles with opposite charges. between the oppositely charged ionElectrically charged particle, formed when an atom or molecule gains or loses electrons.. These forces are usually referred to as ionic bondingIonic bonding forms between two atoms when an electron is transferred from one atom to the other, forming a positive-negative ion pair.. As the ionic lattice contains such a large number of ions, a lot of energy is needed to overcome these ionic bonds so ionic compounds have high melting and boiling points.
| Ionic compound | Melting point | Boiling point |
| NaCl | 801°°ä | 1,413°°ä |
| MgO | 2,852°°ä | 3,600°°ä |
| Ionic compound | NaCl |
|---|---|
| Melting point | 801°°ä |
| Boiling point | 1,413°°ä |
| Ionic compound | MgO |
|---|---|
| Melting point | 2,852°°ä |
| Boiling point | 3,600°°ä |
Ionic bonds between Mg2+ and O2- ions are stronger than those between Na+ and Cl- ions.
Conduction of electricity
A substance can conduct electricity if:
- it contains charged particlesParticles, usually ions or electrons, that carry electrical charges., and
- these particles are free to move from place to place
Ionic compounds conductTo allow electricity, heat or other energy forms to pass through. electricity when moltenA term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid. to form a liquid or dissolved in water to form an aqueousDissolved in water to form a solution. Shown as (aq) in chemical equations. solution. This is because both processes make their ions free to move from place to place. Ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move.
More guides on this topic
- Equations and formulae - Edexcel
- Hazards and risks - Edexcel
- Atomic structure - Edexcel
- The periodic table - Edexcel
- Simple molecular substances - Edexcel
- Giant covalent substances - Edexcel
- Metals and non-metals - Edexcel
- Chemistry calculations - Edexcel
- Mole calculations (higher) - Edexcel
- Sample exam questions - key concepts in chemistry - Edexcel