Mistake Master
Structure of ionic solids
Table salt is not a bunch of NaCl molecules — there is no such molecule. It is a vast 3-D lattice of ions locked together by attractions pulling in every direction. That structure is why ionic solids are hard, brittle, and high-melting.
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Lattices, not molecules.
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An ionic compound is a repeating 3-D lattice of cations and anions, not a collection of separate molecules. In salt, every sodium ion is surrounded by chloride ions and every chloride by sodium ions.
The Coulombic attraction between oppositely charged ions acts in all directions at once. Each ion is held by many neighbors, and the whole network of attractions is what binds the crystal — strongly.
That structure sets the properties. Ionic solids are hard and high-melting because many strong attractions must be overcome, and brittle because shifting the lattice lines up like charges, which then repel and shatter it.
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From lattice to properties.
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Reason each property from the network of attractions, not from a molecule.
- Read the formula as a ratio. NaCl means a 1:1 ratio of Na⁺ to Cl⁻ in the lattice, not a two-atom molecule. The formula is the smallest whole-number ratio, a formula unit.
- Locate every ion's neighbors. Each ion is surrounded by several oppositely charged ions; attractions pull from all sides, giving stability.
- Explain the melting point. High, because melting must overcome many strong Coulombic attractions across the lattice.
- Explain the brittleness. A blow shifts a layer so like charges align; the sudden repulsion cracks the crystal. Strong bonding does not mean bendable.
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The pieces you'll meet.
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An ionic solid's behavior all traces to its lattice.
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Worked example: why does NaCl melt so high yet shatter?
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Question. Sodium chloride melts near 800 °C but shatters under a hammer. Explain both from its structure.
High melting point. In the lattice, every ion is gripped by several oppositely charged neighbors. Melting means overcoming all those strong Coulombic attractions at once, which takes a lot of energy — hence the high temperature.
Brittleness. A hammer blow shoves one layer of ions over by one position. Now cations sit above cations and anions above anions; the like charges repel violently and the crystal splits along that plane.
The lesson. Both facts come from the same strong, directional-in-all-directions attractions — strength and brittleness are not a contradiction.
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Mistakes that cost real points.
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"A formula unit like NaCl is a molecule."
There is no NaCl molecule. NaCl is the ratio of ions (1 Na⁺ : 1 Cl⁻) in an extended lattice. Each ion belongs to many neighbors, not to one partner, so 'a molecule of salt' does not exist.
Fix. Read an ionic formula as the smallest whole-number ratio of ions in the lattice, a formula unit, not a discrete molecule.
"Strong bonds should make ionic solids malleable, like metals."
Strong bonding makes ionic solids hard, but hardness is not malleability. Because the bonds are rigid and directional-in-all-directions, shifting the lattice brings like charges together and it shatters. Metals bend; ionic solids crack.
Fix. Distinguish strength from ductility. Ionic solids are strong but brittle; only the mobile electron sea of a metal allows bending without breaking.
"Melting an ionic solid breaks the ions into atoms."
Melting only frees the ions to move; it does not split them into neutral atoms or break bonds within an ion. Molten salt still contains Na⁺ and Cl⁻ ions, which is why it conducts electricity.
Fix. Picture melting as loosening the lattice so ions flow, keeping their charges. The ions survive melting; only their fixed positions are lost.
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Skill Check.
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