Ionic Bonding
5. Properties of Ionic Compounds
Learning outcomes
- I can explain why ionic compounds have high melting and boiling points.
- I can describe electrical conductivity in molten and aqueous ionic compounds.
- I can explain the brittleness of ionic solids.
- I can relate physical properties to ionic structure.
- I can predict properties based on ionic bonding.
Introduction
The properties of a substance depend on how its particles are arranged and held together.
In the previous lesson, you learned that ionic compounds consist of giant crystal lattices made of positive and negative ions held together by strong electrostatic attractions.
These strong attractions give ionic compounds many distinctive properties, including:
- High melting and boiling points
- Hard but brittle structures
- Electrical conductivity when molten or dissolved in water
- Crystalline solids at room temperature
By understanding the structure of ionic compounds, we can explain why they behave the way they do.
Property 1: High Melting and Boiling Points
Ionic compounds usually have very high melting and boiling points.
For example:
| Compound | Melting Point (°C) |
|---|---|
| Sodium chloride (NaCl) | 801 |
| Magnesium oxide (MgO) | 2852 |
| Calcium oxide (CaO) | 2572 |
Why?
The positive and negative ions are held together by strong electrostatic attractions.
A large amount of energy is required to overcome these attractions.
The stronger the attractions, the higher the melting and boiling points.
Property 2: Hard Solids
Ionic compounds are generally hard solids.
The ions are locked into fixed positions within the crystal lattice.
Since the ions cannot move easily, the solid resists being scratched or compressed.
Examples include:
- Table salt
- Magnesium oxide
- Calcium fluoride
These compounds maintain their shape because the ions are strongly held together.
Property 3: Brittleness
Although ionic compounds are hard, they are also brittle.
If enough force is applied, layers of ions can shift.
When this happens:
- Positive ions may move beside other positive ions.
- Negative ions may move beside other negative ions.
Since like charges repel, the crystal cracks or shatters.
This explains why ionic crystals break rather than bend.
Property 4: Electrical Conductivity
Electrical current is the movement of charged particles.
Since ions are charged, you might expect ionic compounds to conduct electricity.
However, this depends on whether the ions can move.
Ionic solids
In a solid crystal:
- ions are fixed in position
- they cannot move
Result:
Do not conduct electricity
Molten ionic compounds
When melted:
- ions become free to move
Result:
Conduct electricity
Ionic compounds dissolved in water
When dissolved:
- ions separate from one another
- ions move freely through the solution
Result:
Conduct electricity
Why Do Metals Conduct Differently?
It is useful to compare ionic compounds with metals.
| Ionic Compound | Metal |
|---|---|
| Conducts only when molten or dissolved | Conducts as a solid and as a liquid |
| Charge carried by ions | Charge carried by electrons |
The particles responsible for conduction are different.
Property 5: Crystalline Appearance
Many ionic compounds form crystals with:
- flat surfaces
- straight edges
- regular shapes
This happens because the ions are arranged in an orderly repeating lattice.
Examples include:
- Table salt
- Potassium bromide
- Calcium fluoride
Relating Properties to Structure
Every property of an ionic compound can be explained by its structure.
| Property | Structural Explanation |
|---|---|
| High melting point | Strong electrostatic attractions require large amounts of energy to overcome. |
| Hard | Ions are locked into fixed positions. |
| Brittle | Shifting ions brings like charges together, causing repulsion. |
| Conducts when molten | Ions are free to move and carry charge. |
| Conducts in solution | Dissolved ions move freely through water. |
| Does not conduct when solid | Ions cannot move within the lattice. |
Predicting Ionic Properties
If you know a substance has ionic bonding, you can predict that it is likely to:
- have a high melting point
- have a high boiling point
- be hard
- be brittle
- conduct electricity when molten
- conduct electricity when dissolved in water
- not conduct electricity as a solid
- form a crystal lattice
Real-World Applications
The properties of ionic compounds make them useful in many applications.
Examples include:
- Road salt for melting ice
- Electrolysis to produce metals and chemicals
- Ceramic materials
- Electrolyte solutions in medicine
- Batteries
- Water treatment
- Fertilizers
Scientists select ionic compounds because their properties suit particular uses.
Worked Examples
Example 1
Why does sodium chloride have a high melting point?
Answer:
Because strong electrostatic attractions between ions require a large amount of energy to overcome.
Example 2
Why does solid sodium chloride not conduct electricity?
Answer:
Its ions are fixed in the crystal lattice and cannot move.
Example 3
Why does molten sodium chloride conduct electricity?
Answer:
The ions are free to move and carry electric charge.
Example 4
Why are ionic compounds brittle?
Answer:
When layers shift, like charges are brought together. The repulsion between like charges causes the crystal to crack or shatter.
Example 5
Predict two properties of magnesium oxide.
Answer:
Possible answers include:
- High melting point
- Hard solid
- Brittle
- Conducts electricity when molten
- Does not conduct electricity as a solid
Did You Know?
Molten ionic compounds are used in a process called electrolysis to produce important substances such as aluminium, chlorine, and sodium. During electrolysis, the moving ions carry electric current through the molten liquid, allowing chemical reactions to occur that would not be possible in the solid state.
Key Terms
| Term | Definition |
|---|---|
| Melting Point | The temperature at which a solid changes into a liquid. |
| Boiling Point | The temperature at which a liquid changes into a gas. |
| Electrical Conductivity | The ability of a substance to allow electric current to flow. |
| Molten | In the liquid state after being melted. |
| Aqueous Solution | A solution in which a substance is dissolved in water. |
| Brittle | Likely to crack or shatter when force is applied. |
| Crystal Lattice | A giant repeating arrangement of ions held together by strong electrostatic attractions. |
Key Takeaways
- Ionic compounds have high melting and boiling points because strong electrostatic attractions hold the ions together.
- They are hard because the ions are locked into fixed positions within a crystal lattice.
- They are brittle because shifting the lattice brings like charges together, causing repulsion.
- Solid ionic compounds do not conduct electricity because the ions cannot move.
- Molten and aqueous ionic compounds conduct electricity because the ions are free to move.
- The physical properties of ionic compounds can be explained by their giant crystal lattice structure and ionic bonding.