Answer:
Chlorine
Explanation:
The two halogens that are gases at room temperature are Fluorine and Chlorine, as they are at the top of the group.
The heaviest of the two is chlorine as the density of Fluorine is 1.696g/dm3 and the density of Chlorine is 3.214g/dm3 so chlorine will weigh more with the same amount.
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