Ionic crystal structures are orderly three-dimensional arrangements of positive and negative ions. They matter because the pattern of ions controls many properties, including melting point, hardness, brittleness, and electrical conductivity. Instead of existing as separate molecules, ionic compounds form repeating lattices that extend through the whole solid.
Understanding these patterns helps explain why salts such as NaCl and CsCl have different crystal shapes and behaviors.
The basic repeating block of a crystal is the unit cell, which can be translated in all directions to build the full lattice. Coordination number tells how many oppositely charged ions directly surround a given ion, and it depends on ion sizes and packing geometry. Common ionic structures include rock salt, where each ion has 6 nearest oppositely charged neighbors, and cesium chloride, where each ion has 8.
The strong electrostatic attractions throughout the lattice give ionic solids high melting points, but when layers shift, like charges can line up and the crystal can crack.
Key Facts
- Coulomb attraction holds ionic lattices together: F = k|q1q2|/r^2.
- A unit cell is the smallest repeating 3D block that shows the symmetry and ion arrangement of a crystal.
- Coordination number is the number of nearest oppositely charged ions around one ion.
- Rock salt structure, such as NaCl, has coordination number 6:6.
- Cesium chloride structure, CsCl, has coordination number 8:8.
- Lattice energy generally increases when ion charges are larger and ion distances are smaller.
Vocabulary
- Ionic lattice
- An ionic lattice is a repeating three-dimensional arrangement of cations and anions held together by electrostatic attraction.
- Unit cell
- A unit cell is the smallest repeating part of a crystal that can be stacked to reproduce the whole structure.
- Coordination number
- Coordination number is the number of nearest neighboring ions of opposite charge around a chosen ion.
- Cation
- A cation is a positively charged ion formed when an atom or group of atoms loses electrons.
- Lattice energy
- Lattice energy is the energy change associated with forming one mole of an ionic solid from its separated gaseous ions.
Common Mistakes to Avoid
- Treating an ionic solid as separate NaCl molecules is wrong because the solid is a continuous lattice of many ions, not independent molecular units.
- Counting same-charge neighbors for coordination number is wrong because coordination number counts only the nearest ions of opposite charge.
- Assuming all ionic crystals have the same unit cell is wrong because ion size ratio and charge balance can lead to different structures such as rock salt, cesium chloride, or zinc blende.
- Thinking ionic solids conduct electricity well in the solid state is wrong because the ions are locked in fixed positions and cannot move freely until melted or dissolved.
Practice Questions
- 1 In a rock salt structure, each Na+ ion is surrounded by 6 Cl- ions. What is the coordination number of Na+ and what is the coordination number of Cl-?
- 2 A CsCl unit cell has one Cs+ ion at the center and Cl- ions at the 8 corners. Since each corner ion contributes 1/8 to the unit cell, how many Cl- ions are counted per unit cell, and what is the Cs:Cl ratio?
- 3 Explain why an ionic crystal is brittle even though the electrostatic attractions between its ions are strong.