Giant ionic lattices: strong forces pull oppositely charged ions together in every direction
Definition
Ionic compound
A compound formed from positively charged ions and negatively charged ions with no overall electrical charge.
Definition
Ion
An atom or group of atoms with an overall electrical charge because electrons have been lost or gained.
- An ionic compound is built as a giant ionic lattice, a regular, repeating arrangement of a very large number of ions.
- The lattice contains positive ions and negative ions, each surrounded by ions of the opposite charge.
- In sodium chloride, Na+\text{Na}^+Na+ and Cl−\text{Cl}^-Cl− ions alternate so that every ion touches neighbours of opposite charge.
- Strong electrostatic forces of attraction act between the oppositely charged ions.
- These attractions act in all directions throughout the lattice, so the whole structure is held together very strongly.
Note
- Sodium chloride, NaCl\text{NaCl}NaCl, is the usual example of a giant ionic lattice.
- You do not need to recall the detailed structures of any ionic compounds other than sodium chloride.
High melting and boiling points: many strong bonds need a lot of energy to break
Definition
Melting point
The melting point is the temperature at which a substance changes from a solid to a liquid.
- Heating an ionic solid transfers energy to the ions held in its lattice.
- Melting or boiling the compound means overcoming many of the strong electrostatic attractions between oppositely charged ions.
- A large amount of energy is required because the attractions are strong and extend throughout the whole giant lattice.
- Ionic compounds therefore have high melting points and high boiling points.
Common Mistake
- Do not explain the high melting point using intermolecular forces, because ionic compounds are not made of molecules.
- Link the property to the energy needed to overcome many strong electrostatic attractions between oppositely charged ions.
Electrical conduction: ions carry charge only when they are free to move
Definition
Molten
Heated until it has melted into a liquid.
- An electric current is a flow of charge through a substance.
- In a solid ionic compound, the ions are locked in fixed positions within the lattice.
- A solid ionic compound therefore does not conduct electricity, because its charged ions cannot move through the structure.
- When the compound is molten, the lattice has broken apart and the ions become free to move.
- When the compound is dissolved in water, the ions separate and move freely through the solution.
- Molten or dissolved ionic compounds conduct electricity because their moving ions carry the charge.
- The charge is carried by moving ions, not by moving electrons.
Example
- Solid sodium chloride, NaCl(s)\text{NaCl}(\text{s})NaCl(s), does not conduct electricity because its ions are fixed in the lattice.
- Molten sodium chloride, NaCl(l)\text{NaCl}(\text{l})NaCl(l), and sodium chloride solution conduct electricity because their ions are free to move.
Explaining the properties: connect the lattice to energy or to ion movement
- To explain a high melting or boiling point, say that the giant ionic lattice holds many strong electrostatic attractions that need a large amount of energy to overcome.
- To explain electrical conduction, state whether the ions are fixed or free to move, then link their movement to the flow of charge.
- Remember that ionic compounds conduct when molten or dissolved in water, but not when solid.
Self review
- What type of structure does an ionic compound have?
- What holds oppositely charged ions together in an ionic lattice?
- Why do ionic compounds have high melting and boiling points?
- Why does a solid ionic compound not conduct electricity?
- Why can an ionic compound conduct electricity when molten or dissolved in water?
