| Property | Across a Period → | Down a Group ↓ | Driving Force |
|---|---|---|---|
| Atomic Radius | Decreases | Increases | Zeff ↑ across; new shells added down |
| 1st Ionisation Energy | Increases | Decreases | Higher Zeff grips e⁻ tighter; more shielding down |
| Electron Affinity | More exothermic | Less exothermic | Incoming e⁻ pulled in more strongly with higher Zeff |
| Electronegativity | Increases | Decreases | Stronger pull on shared e⁻ pair across; weaker down |
🔑 The Universal Explanation
All four trends are driven by the same two factors: effective nuclear charge (Zeff) and shielding. Across a period, Zeff increases (more protons, same shielding). Down a group, more electron shells = more shielding and greater distance.
⚠️ Examiner Trap. IE Definition
First ionisation energy is "the minimum energy required to remove one mole of electrons from one mole of gaseous atoms." If you omit the word "gaseous", you will lose the mark.
🔑 Mark-Scoring Tip – Comparing Two Elements
When asked to compare a property between two elements, you must mention BOTH atoms in your answer. E.g. "Na has a lower IE than Cl because Na has a larger atomic radius and fewer protons, so its outer electron experiences less Zeff." Leaving one atom implied does not earn the mark.