Fluorine is a gas that is considered to be the most active element, chemically speaking. It's the most active because it displaces other halogens from their compounds - fluorine even displaces oxygen from water.
Fluorine is a pale, greenish-yellow gas, slightly heavier than air, poisonous, corrosive, and of penetrating and disagreeable odour. Its atomic weight is 18.998. Fluorine melts at -219.61° C (-363.30° F), boils at -188.13° C (-306.63° F), and has a relative density of 1.51 in its liquid state at its boiling point. It is the most chemically active of the non-metallic elements. It combines directly with most elements and indirectly with nitrogen, chlorine, and oxygen. Nearly all compounds are decomposed by fluorine to form fluorides that are among the most stable of all chemical compounds.