AP Chemistry Unit 2 Study Notes

AP Chemistry 2.3: Structure of ionic solids

Connect lattice structure to ionic compound properties.

Aligned to Compound Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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3.1 Ionic Crystals 3.2 Why the Lattice Forms 3.3 Example: NaCl 3.4 Coulomb's Law and Ionic Attraction 3.5 Effect of Ionic Charge 3.6 Effect of Ionic Size 3.7 Lattice Energy 3.8 Comparing Ionic Compounds 3.9 Another Comparison 3.10 Properties of Ionic Solids 3.11 Why Ionic Solids Are Brittle 3.12 Electrical Conductivity of Ionic Substances 3.13 Molten Ionic Compounds 3.14 Ionic Compounds in Water Topic 3 Quick Rule

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3.1 Ionic Crystals Open

  • Ionic solids consist of:

    • cations + anions

    • arranged in a repeating three-dimensional lattice.

    • They do not consist of individual molecules.

3.2 Why the Lattice Forms Open

  • Opposite charges attract.

  • The lattice arranges ions so that:

    • positive ions are near negative ions

    • while:

    • like-charged ions stay as separated as possible.

    • This lowers the electrostatic potential energy.

3.3 Example: NaCl Open

  • NaCl contains:

    • Na⁺

    • and:

    • Cl⁻

    • in a 1:1 ratio.

  • In the solid, one Na⁺ interacts with several Cl⁻ ions rather than being paired with one specific chloride.

3.4 Coulomb's Law and Ionic Attraction Open

  • Remember from Unit 1:

    • F ∝ |q₁q₂| / r²

    • This tells us ionic attraction depends strongly on:

    • charge

    • distance

3.5 Effect of Ionic Charge Open

  • Compare:

    • Na⁺ and Cl⁻

    • versus:

    • Mg²⁺ and O²⁻

  • Mg²⁺ and O²⁻ have greater charge magnitudes.

    • Therefore their electrostatic attractions are generally stronger.

  • Stronger attractions usually lead to:

    • greater lattice energy magnitude

    • higher melting point

    • more energy required to separate ions

3.6 Effect of Ionic Size Open

  • Smaller ions can approach each other more closely.

    • Remember:

    • smaller r → stronger Coulombic attraction

    • Therefore, when charges are similar:

    • smaller ions generally produce stronger ionic attractions

3.7 Lattice Energy Open

  • Lattice energy is related to the energy involved in separating or forming an ionic lattice.

  • Depending on the definition being used, the sign may be discussed differently.

  • For AP reasoning, focus on the magnitude of the attraction .

  • Greater lattice-energy magnitude means:

    • stronger ion-ion attraction

3.8 Comparing Ionic Compounds Open

  • Suppose we compare:

    • NaCl

    • MgO

  • MgO contains:

    • Mg²⁺ and O²⁻

  • NaCl contains:

    • Na⁺ and Cl⁻

  • The charge product in MgO is much greater.

    • Therefore:

  • MgO has substantially stronger electrostatic attractions.

3.9 Another Comparison Open

  • Compare:

    • LiF

    • CsF

  • Both have:

  • +1 and −1 charges.

    • So charge is essentially the same factor.

  • Now compare ion size.

  • Li⁺ is much smaller than Cs⁺.

    • Therefore Li⁺ and F⁻ can get closer.

    • So:

  • LiF generally has stronger ionic attraction than CsF.

3.10 Properties of Ionic Solids Open

  • Strong ionic attractions help explain why ionic compounds tend to have:

    • high melting points

    • high boiling points

    • hard crystal structures

  • A lot of energy is required to separate the ions.

3.11 Why Ionic Solids Are Brittle Open

  • Ionic crystals are hard but often brittle .

  • If enough force shifts one layer of ions:

    • positive ions may become aligned with positive ions.

  • Negative ions may become aligned with negative ions.

  • Like charges repel strongly.

  • The crystal can crack or shatter.

3.12 Electrical Conductivity of Ionic Substances Open

  • Solid ionic compounds generally do not conduct electricity well .

    • Why?

  • The ions are locked into fixed positions.

  • Charged particles cannot freely move through the solid.

3.13 Molten Ionic Compounds Open

  • When melted:

    • the ions become mobile.

    • Therefore molten ionic compounds:

    • can conduct electricity

3.14 Ionic Compounds in Water Open

  • If an ionic compound dissolves in water:

    • its ions may separate and move through the solution.

    • Therefore aqueous solutions containing mobile ions:

    • can conduct electricity

    • This becomes very important later when studying electrolytes.

Topic 3 Quick Rule Open

  • Ionic attraction gets stronger when:

    • charge magnitude ↑

    • or:

    • ionic distance ↓