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2.3Compound Structure and Properties

Structure of Ionic Solids — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 9 of 13 1 quiz

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2.3 Structure of Ionic Solids — Notes — AP CHEMISTRY notes: Describe the structure of ionic solids and explain their properties using Coulomb's law.An ionic solid is a three-dimensional periodic lattice of alternating cations and anions.The arrangement maximises attractions and m…

  • Formula unit
  • Ion-dipole interaction
  • The arrangement maximises attractions and minimises repulsions between like charges
  • Knowledge of specific crystal structures is not assessed on the AP exam
  • Ionic solids have high melting points because many strong Coulombic attractions must be overcome
  • Melting point increases with greater ion charge and smaller ionic radii

Section 1 of 9Structure of an ionic solid

1
An ionic solid is a three-dimensional periodic lattice of alternating cations and anions.
2
The arrangement maximises attractions and minimises repulsions between like charges.
3
Each ion is surrounded by ions of opposite charge, and there are no discrete molecules, so formulae are formula units.
4
Knowledge of specific crystal structures is not assessed on the AP exam.

Section 2 of 9Melting point

Melting point
Ionic solids have high melting points because many strong Coulombic attractions must be overcome.
Melting point increases with greater ion charge and smaller ionic radii.
CompoundIon chargesMelting point
NaCl+1/−1801 °C
KCl+1/−1 (larger K+)770 °C
CaF2+2/−11418 °C
MgO+2/−22852 °C

Section 3 of 9Mechanical properties

Mechanical properties
Ionic solids are hard but brittle.
When force shifts a layer, like charges align and repel, so the crystal cleaves.

Section 4 of 9Electrical conductivity and solubility

Electrical conductivity and solubility
Solid ionic compounds do not conduct because ions are fixed in position.
Molten or dissolved ionic compounds conduct because the ions are free to move.
Many are soluble in water because ion-dipole attractions compensate for breaking the lattice.

Section 5 of 9Particle diagrams

Particle diagrams
Show alternating positive and negative ions in a three-dimensional pattern.
The ratio of ions must match the formula.
The larger ion is usually the anion, for example Cl− larger than Na+.

Section 6 of 9Comparing melting points

1
Identify the ions and their charges.
2
Compare charge magnitudes; the higher charge gives stronger attraction.
3
If charges are equal, compare ionic radii using periodic trends.
4
Apply Coulomb's law, F ∝ q1q2/r2.
5
Conclude that stronger attraction needs more energy, giving a higher melting point.

Section 7 of 9More from the specification

More from the specification
Substitutional alloys in metallurgy
  • Substitutional alloys in metallurgy solute atoms of comparable atomic radius substitute directly into host lattice sites of base metals,
    • forming substitutional alloys.
Interstitial alloys crystal structures
  • Interstitial alloys crystal structures small non-metal atoms pack into spaces between larger host atoms of transition metals to produce interstitial alloys.
Interstitial alloy
  • Alloy in which small atoms occupy holes in the host lattice.
Ionic radius
  • Distance from the nucleus to the outermost electrons of an ion.
Substitutional alloy
  • Alloy in which atoms of similar size replace host atoms.
Physical property
  • A property observed or measured without changing the material.
Alloy
  • A mixture of a metal with one or more other elements.
Metal
  • An element with a lattice of positive ions in a sea of delocalised electrons, usually shiny, conductive and malleable.
Electron
  • Negative particle moving around the nucleus.
Bonding
  • The joining of atoms.

Section 8 of 9Key terms

TermMeaning
Ionic solidLattice of alternating cations and anions.
LatticeRepeating three-dimensional arrangement of particles.
BrittleBreaks when layers shift and like charges repel.
Formula unitSimplest ratio of ions in an ionic compound.
Ion-dipole interactionAttraction between an ion and a polar molecule.
Coulomb's lawAttraction proportional to charges and inversely related to distance.

Section 9 of 9Exam technique

1
Saying ionic solids conduct electricity as solids — they conduct only when molten or dissolved
2
Explaining brittleness by weak bonds — layers shift and like charges repel
3
Ignoring ion size when comparing lattices of equal charge
4
Drawing molecules in an ionic solid — it is a continuous lattice

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