In general, to determine which is more reactive, a good place for you to start is the octet rule. This rule applies for main group elements where the valence or outer shell consists of s and p electrons. This shell is filled when there are 8 electrons in it. Elements will react to fill their valence shell because a filled valence shell is more stable than an unfilled shell. We can see from a periodic table that krypton has a filled valence shell, since it is in group 18 and on the far right of the periodic table. Group 18 is known as the noble gases. They are considered noble because they do not react with other elements in normal conditions due to their filled valence shell. Bromine is in group 17, the halogens, and so it has one unfilled spot in its valence shell. Therefore, bromine is more reactive than krypton.