(2) The distribution of diffracting power in certain crystals. Potassium and ammonium iodide have the sodium chloride structure; ammonium chloride has the body-centred cæsium chloride structure.

Dot and cross diagrams help us to model when ions are formed from atoms. Here’s an example using sodium and chlorine. They form ions which bond to form sodium chloride. 2. Work out how many ...