Structure of Ionic Solids
Introduced| English | Meaning |
|---|---|
| crystal lattice/ˈkrɪstl ˈlætɪs/ | A regular repeating three-dimensional arrangement of positions describing a crystalline solid. |
| lattice energy/ˈlætɪs ˈenədʒi/ | The enthalpy change associated with forming an ionic solid from gaseous ions, or the reverse process under a stated sign convention. |
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.
Ions pack into a giant lattice of alternating positive and negative ions
In an ionic lattice, each ion is surrounded by ions of the opposite charge.
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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?
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Larger charges attract far more strongly, so MgO has the higher lattice energy.
A higher lattice energy generally means a lower melting point.
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False. More lattice energy holds the ions together more strongly, so more heat is needed to break the lattice: a higher melting point.
Lattice energy increases when the ion charges get ____.
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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...
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Ions must be free to move -- that happens when melted or dissolved.
Why are ionic solids brittle?
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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.