Compound of 弗 (futsu, used phonetically) + 素 (so, “chemical element”). 弗 (futsu) was chosen for the initial fu, to match the initial /f/ in English fluorine.
フッ素 (hiragana and katakana フッそ, rōmaji fusso)
- fluorine, F
- fluorine gas
Commonly written with katakana instead of kanji for the 弗 portion, due to the rarity of the 弗 character.