Structure of Ionic Solids
Introduced| English |
|---|
| crystal lattice/ˈkrɪstl ˈlætɪs/ |
| lattice energy/ˈlætɪs ˈenədʒi/ |
A perfect repeating grid
- Table salt crystals have flat faces and sharp corners.
- Zoom in and you find a grid that repeats forever.
- Positive and negative charges alternate like a 3D checkerboard.
- That orderly pattern gives salt its tough, brittle nature.
The lattice
- An ionic solid is a crystal lattice 晶格 of alternating positive and negative ions.
- Each ion is surrounded by neighbours of the opposite charge.
- Strong electrostatic attraction locks the whole grid together.
In an ionic lattice, each ion is surrounded by ions of the opposite charge.
Alternating opposite charges maximise attraction and stability.
Lattice energy
- The lattice energy 晶格能 is the energy holding the lattice together.
- Higher charges and smaller ions give stronger attraction and higher lattice energy.
- This is what sets the melting point.
Which ionic compound has the higher lattice energy?
Larger charges attract far more strongly, so MgO has the higher lattice energy.
A higher lattice energy generally means a higher melting point.
More energy holds the lattice, so more heat is needed to break it apart.
Lattice energy increases when the ion charges get ____.
Stronger charges attract more, raising the lattice energy.
Why ionic solids behave as they do
- High melting points: the strong grid is hard to break apart.
- Brittle: shift a layer and like charges meet and repel, cracking it.
- They conduct only when molten or dissolved, when the ions can move.
Inside an ionic solid
Oppositely charged ions pack into a giant repeating lattice held by electrostatic forces.
An ionic solid conducts electricity...
Ions must be free to move -- that happens when melted or dissolved.
Why are ionic solids brittle?
A small shift brings like charges together; their repulsion cracks the crystal.
Which has the higher lattice energy, $\text{NaCl}$ or $\text{MgO}$?
- MgO has $2+$ and $2-$ ions; NaCl has $1+$ and $1-$.
- Bigger charges mean much stronger attraction, so MgO wins and melts far hotter.
Ionic solids do not conduct electricity as solids -- the ions are locked in place; they conduct only once melted or dissolved. Higher ion charge raises lattice energy far more than small size differences do. And the brittleness comes from the lattice: a small shift lines up like charges, which repel and shatter it.
An ionic solid is a crystal lattice of alternating ions held by strong electrostatic attraction. Its lattice energy -- larger for higher charges and smaller ions -- gives it a high melting point. It is brittle and conducts only when molten or dissolved, once the ions are free to move.