human
What is characteristic of a halogen, and how does the activity of halogens change in a group?
gpt
A halogen can replace another halogen from a compound, and the activity of halogens decreases going down.
Explanation: In a group of the periodic table, halogens can replace elements below them because they are more reactive. However, the reactivity or activity of halogens, specifically their tendency to replace elements in compounds, decreases moving downwards.