乙烯

乙烯 = 乙 (Heavenly Stem #2 / 'eth-') + 烯 (alkene suffix). Chinese systematic chemistry uses the ten Heavenly Stems (甲乙丙丁) for carbon counts: 甲烷 = methane (1C), 乙烷 = ethane (2C), 乙烯 = ethene (2C with double bond). Once you learn the system, organic-chemistry vocabulary unlocks fast.

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烯 (xī), fire radical 火 plus the rare 希 phonetic, is the dedicated suffix for alkenes (the '-ene' of English). Pair with 烷 (-ane) and 炔 (-yne) for the full set.