Explain how atomic size and metallic character change (a) across a period and (b) down a group. Give reasons and an example of each.

1. 1 · Define. Atomic size is the distance from the nucleus to the outermost shell; metallic character is the tendency to lose electrons.
2. 2 · Across: size. Decreases: the nuclear charge rises but the shell stays the same, so electrons are pulled in (Li 134 pm to F 72 pm).
3. 3 · Across: metallic character. Decreases: electrons are held more tightly (Na, Mg and Al are metals; Si a metalloid; P, S and Cl non-metals).
4. 4 · Down: size. Increases: a new shell is added at each step (Li 134 pm to Cs 225 pm).
5. 5 · Down: metallic character. Increases: outer electrons are farther away and lost more easily (C is a non-metal, Pb a metal in group 14).
6. 6 · Real life. Potassium reacts with water more violently than sodium.
7. 7 · Close. So size and metallic character move together: both fall across a period and rise down a group.

Scoring tip: Each trend needs its reason in a few words: 'more nuclear charge' or 'a new shell added'.