Chapter summary
The Periodic Table, at a glance
Use electron arrangements and structure to explain trends, reactions and unknown elements.
A-Level 8873, revised syllabus (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Atomic radius | Half the distance between the nuclei of two neighbouring atoms of the same element. |
| Shielding | The repulsion of the outer electrons by inner-shell electrons, which reduces the attraction the outer electrons feel from the nucleus. |
| First ionisation energy | The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous singly charged positive ions: M(g) → M+(g) + e-. |
| Electronegativity | The ability of an atom in a molecule to attract the shared pair of electrons in a covalent bond towards itself. |
| Property | Main cause |
|---|---|
| Atomic radius / first IE | Nuclear charge, shielding and occupied shell; pairing/subshell exceptions. |
| Halogen boiling/volatility | Intermolecular attractions increase with polarisability. |
| Hydrogen-halide thermal stability | H-X covalent bond energy falls down the group. |
| Group 1 reducing behaviour | Ease of losing the outer electron increases down Li-Cs. |
| Group 17 oxidising behaviour | Ease of accepting electrons decreases down Cl-I. |
| Period 3 oxide character | Basic → amphoteric → acidic; insolubility is not neutrality. |
| Chloride solution pH | Distinguish simple dissolution, hydrated-cation hydrolysis and molecular hydrolysis. |
Quick recall
Why does atomic radius decrease across Period 3?