# Periodicity (IB)
Periodicity is the regular repeating pattern of physical and chemical properties across periods in the periodic table. Understanding these trends is fundamental to predicting element behaviour.
1. Trends Across Period 3
| Property | Trend Left → Right | Explanation |
|---|---|---|
| Atomic radius | Decreases | More protons, greater nuclear charge, electrons pulled closer |
| Ionic radius | Cations decrease; anions are larger than cations | Cations lose shells; anions gain electrons |
| Ionisation energy | Generally increases (with anomalies) | Greater nuclear charge; same shielding |
| Electronegativity | Increases | Stronger attraction for bonding electrons |
| Electron affinity | Generally more negative | Atoms more readily accept electrons |
IE Anomalies
- Be → B: B has electron in 2p (higher energy, easier to remove)
- N → O: O has paired 2p electron (repulsion makes it easier to remove)
2. Trends Down a Group
| Property | Trend |
|---|---|
| Atomic radius | Increases (more shells) |
| IE | Decreases (more shielding, greater distance) |
| Electronegativity | Decreases |
| Metallic character | Increases |
3. Oxides of Period 3
| Oxide | Na₂O | MgO | Al₂O₃ | SiO₂ | P₄O₁₀ | SO₃ |
|---|---|---|---|---|---|---|
| Bonding | Ionic | Ionic | Ionic/covalent | Giant covalent | Covalent | Covalent |
| With water | Alkaline | Weakly alkaline | Amphoteric | Insoluble | Acidic | Acidic |
Trend: basic → amphoteric → acidic across the period.
4. Transition Metals (HL)
At HL, periodicity extends to d-block elements:
- Variable oxidation states
- Coloured compounds (d-d transitions)
- Catalytic activity
- Complex ion formation
5. Practice Questions
- Explain the trend in atomic radius across Period 3.
- Why is the first IE of aluminium lower than magnesium?
- Describe and explain the acid-base character of Period 3 oxides.
- Predict properties of an element in Group 2, Period 4.
- Compare the properties of Na₂O and SO₃ when added to water.
Want to check your answers and get step-by-step solutions?
Summary
- Across a period: atomic radius ↓, IE ↑, EN ↑
- Down a group: atomic radius ↑, IE ↓, EN ↓
- Period 3 oxides: basic (metals) → amphoteric (Al) → acidic (non-metals)
- IE anomalies explained by subshell energies and electron pairing