Explain how atomic radius, ionisation energy, electron affinity and electronegativity vary along a period and down a group.

1. 1 · Open with the idea. Periodic properties change regularly across a period and down a group because nuclear charge and the number of shells change.
2. 2 · Atomic radius. Distance from the nucleus to the outermost shell. Decreases across (more nuclear charge, same shell); increases down (new shells added).
3. 3 · Ionisation energy. Minimum energy to remove an electron from an isolated gaseous atom. Increases across; decreases down as the electron is farther and shielded.
4. 4 · Electron affinity. Energy released when an electron is added to an isolated gaseous atom. Increases across; decreases down. Halogens are highest.
5. 5 · Electronegativity. Tendency to attract a shared pair of electrons. Increases across; decreases down. Fluorine is the highest.
6. 6 · Draw a trend chart. [A box for the table: an arrow across labelled 'radius decreases; IE, EA, EN increase' and an arrow down labelled 'radius increases; IE, EA, EN decrease']
7. 7 · Real-life link. Sodium's low ionisation energy and chlorine's high electron affinity explain why they join as common salt.
8. 8 · Close in one line. Thus small atoms on the right hold and attract electrons strongly, while large atoms at the bottom left do not.

Scoring tip: For each property write the definition, the trend in both directions and a one-line reason. A labelled arrow chart helps.