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 compared to iodine, which means its outer shell electrons are closer to the nucleus. This results in a stronger electrostatic attraction between the outer electrons and the nucleus. Consequently, chlorine can gain an electron more easily than iodine, making it more reactive. Additionally, iodine has more electron shells, which increases electron shielding, reducing the effective nuclear charge felt by the outer electrons.

The white crystals formed in the gas jar are sodium chloride (NaCl), commonly known as table salt, produced by the reaction between sodium and chlorine.