Properties of ionic compounds
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). have regular structures, called giant ionic latticeThe regular arrangement of ions in an ionic substance.. In a giant ionic lattice, there are strong electrostatic forces of attraction acting in all directions between the oppositely charged ions. The structure and bonding of ionic compounds explain their propertiesThe characteristics of something. In chemistry, chemical properties include the reactions a substance can take part in. Physical properties include colour and boiling point..
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High melting points and boiling points
See the study guide on the three states of matter to see how bonding and structure are related to the properties of substances.
Energy must be transferred to a substance to make it melt or boil. This energy overcomes the strong electrostatic forces of attraction which act in all directions between the oppositely charged ions:
- some forces are overcome during melting
- all remaining forces 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.. Since the electrostatic forces of attraction between oppositely charged ions are strong, their melting and boiling points are high.
Explanation
Ionic compounds are held together by 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 latticeThe regular arrangement of ions in an ionic substance. contains such a large number of ions, a lot of energy is needed to overcome this ionic bonding so ionic compounds have high melting and boiling points.
The strength of the ionic bonds depends on the charge on the ions. Ions with higher charge will have stronger forces between them, so will need more energy in order to overcome these forces.
| Compound | Melting point | Boiling point |
| NaCl | 801°C | 1,413°C |
| MgO | 2,852°C | 3,600°C |
| Compound | NaCl |
|---|---|
| Melting point | 801°C |
| Boiling point | 1,413°C |
| Compound | MgO |
|---|---|
| Melting point | 2,852°C |
| Boiling point | 3,600°C |
Ionic bonds between Mg2+ and O2- ions are stronger than those between Na+ and Cl- ions
Conducting electricity
A substance can conductTo allow electricity, heat or other energy forms to pass through. electricity if:
- it contains charged particlesParticles, usually ions or electrons, that carry electrical charges., such as ions, and
- these particles are free to move from place to place
An ionic compound can conduct electricity when:
- it has meltingThe process that occurs when a solid turns into a liquid when it is heated. to form a liquid, or
- it has dissolved in water to form an aqueousDissolved in water to form a solution. Shown as (aq) in chemical equations. solution
Both these processes allow ions to move from place to place. Ionic compounds cannot conduct electricity in the solid state because their ions are held in fixed positions and cannot move.
