Write the electronic configurations of nitrogen (Z = 7) and sodium (Z = 11). Draw the structure of a sodium atom and find the valency of each element.

1. 1 · Rule. Fill K (2) first, then L (8), then M; an outermost shell holds at most 8.
2. 2 · Nitrogen. 7 electrons: 2, 5. It has 5 valence electrons, so valency = 8 − 5 = 3.
3. 3 · Sodium. 11 electrons: 2, 8, 1. It has 1 valence electron, so valency = 1.
4. 4 · Draw sodium. Nucleus with 11 p and 12 n; K with 2, L with 8 and M with 1 electron, all labelled.
5. 5 · Daily life. Sodium's single valence electron is why it combines with chlorine to make common salt.
6. 6 · Close. So the valence electrons decide how an element combines.

Scoring tip: Write the shell names K, L, M above the numbers; the diagram must show electrons adding up to Z.