The 气 radical signals it's a gas element. The right component 弗 (fú) gives the pronunciation. Most gas elements in Chinese use 气 as a radical: 氢 (hydrogen), 氧 (oxygen), 氮 (nitrogen), 氟 (fluorine).
The 气 wrapper marks 氟 as a gaseous element. It heads a coined chemistry family — 氢 hydrogen, 氧 oxygen, 氮 nitrogen, 氦 helium — where 气 frames a phonetic indicating which gas. Here the gas is fluorine.
Tucked inside the 气 corner, 弗 supplies the sound fú with the tone preserved. Its classical meaning of negation is irrelevant here — modern coiners chose it purely to approximate the international name fluorine.
