I am a halogen. I am a gas at room temperature. And of those halogens that are gases, I am the heaviest of the two
Who am I?

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.

Hope this helps, let me know if you have any questions :)

some sources quote the lethal dose, for an average 170lbs opiate-naive individual is between 75 and 375 mg