Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- An ionic solid is a repeating three-dimensional array of cations and anions that extends to the surface of the crystal.
- Each ion is surrounded by ions of the opposite charge. This gives the most attraction and keeps like charges apart.
- There are no molecules. The formula (NaCl, CaF₂) gives only the ratio of ions; one set of ions in that ratio is a formula unit.
- Specific crystal structures are not assessed. You need the general model, not names of structures.
Key relationships
| Idea | What to use | Effect |
|---|---|---|
| Strength of attraction | ∝ |q₊ × q₋| / d, with d ≈ r₊ + r₋ | Larger charges or shorter d give stronger attraction |
| Charge effect | 2+/2− vs 1+/1− | Product of charges is 4 times larger: the biggest effect |
| Size effect (equal charges) | Ion radius down a group increases | Smaller ions: shorter d, stronger attraction |
| Isoelectronic ions | Same electrons, more protons | Smaller ion (Mg²⁺ < Na⁺; F⁻ < O²⁻) |
Rules for a particulate drawing
- Alternate + and −; like charges never touch.
- Cations usually smaller than anions (for example K⁺ 138 pm, Cl⁻ 181 pm).
- Match the formula ratio; the total charge of the drawing is zero.
- Label charges with signs or symbols, not colour alone.
Assumptions
- Ions are treated as hard spheres that touch their opposite-charge neighbours.
- |q₊q₋| / d is for ranking similar solids; it does not give a melting point.
- Radii from different tables differ slightly; trends do not.
Mistakes to avoid
- Drawing ion pairs or “NaCl molecules”.
- Saying bigger ions attract more strongly.
- Ranking a 1+/1− solid above a 2+/2− solid because of a small size difference.
- Claiming the solid conducts because it “has ions”. The ions must be mobile (molten or dissolved).
- Forgetting the ratio: CaF₂ needs two F⁻ for each Ca²⁺.
Quick self-check
- Which has the shorter distance between ion centres, LiF or LiI? (LiF: 76 + 133 = 209 pm, against 296 pm for LiI)
- In a drawing of CaF₂ with 5 Ca²⁺ ions, how many F⁻ ions should there be? (10)
- Why does an ionic crystal shatter when a layer slides? (Like charges line up and repel.)
Next: practice questions.