The elements in group 7 of the periodic table are known as the halogens - Edexcel - GCSE Chemistry - Question 6 - 2022 - Paper 1

Chlorine is more reactive than iodine due to several factors:

1. Atomic Size: Chlorine has a smaller atomic radius compared to iodine, meaning the outer electrons are closer to the nucleus. This results in a stronger attraction between the nucleus and the outer electrons in chlorine.

2. Electron Shells: Chlorine has fewer electron shells than iodine. Being in a higher period, iodine has more shells, which can contribute to a decrease in nuclear attraction on the outer electrons because they are shielded by the inner electron shells.

3. Electronegativity: Chlorine has a higher electronegativity, which allows it to attract electrons more effectively than iodine during reactions.

These factors combined result in chlorine being a more reactive halogen.

The white crystals coating the inside of the gas jar are sodium chloride (NaCl), formed from the reaction between sodium and chlorine gas.