The elements in group 7 of the periodic table are known as the halogens - Edexcel - GCSE Chemistry Combined Science - Question 4 - 2022 - Paper 1

Chlorine is more reactive than iodine due to its atomic structure. Chlorine has fewer electron shells, which allows its outer electrons to be closer to the nucleus. This proximity leads to a stronger attraction between the nucleus and the outer electrons, making it easier for chlorine to gain an electron compared to iodine, which has more electron shells that increase the distance from the nucleus. Therefore, chlorine effectively gains an electron more readily than iodine, enhancing its reactivity.

The white crystals formed at the end of the reaction are sodium chloride (NaCl).