| 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 Periodic Explanation
All four periodic trends are driven by the same two fundamental factors: effective nuclear charge (\(Z_{\text{eff}}\)) and shielding. Across a period, \(Z_{\text{eff}}\) increases (more protons, same inner shielding shells). Down a group, additional electron shells cause increased shielding and greater distance from the nucleus.
Always Specify Gaseous State in IE Definitions
First ionisation energy is "the minimum energy required to remove one mole of electrons from one mole of gaseous atoms under standard conditions." Omitting "gaseous" or omitting the mole concept is an automatic mark loss.
Always Explicitly Compare Both Elements
When asked to compare a property between two elements, you must explicitly state properties for BOTH atoms in your answer (e.g. "Na has a lower IE than Cl because Na has fewer protons and a larger radius, so its valence electron experiences lower \(Z_{\text{eff}}\)"). Leaving one atom implied does not score comparison marks.
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