氯 combines the gas radical 气 with 录 as a phonetic component. Many chemical element characters for gases use the 气 radical: 氧 (oxygen), 氢 (hydrogen), 氮 (nitrogen).
气 wraps the upper-right corner and marks 氯 as a gas. It groups this chemical char with 氧 (oxygen), 氢 (hydrogen), 氮 (nitrogen) and 氟 (fluorine) - the 气-radical naming pattern adopted in the modern Chinese chemistry vocabulary.
录 lù gives the sound, drifted to lǜ in 氯. Picked because of the sound match - 氯 was coined for chlorine partly to echo the chlor- root and partly because Cl gas is a sickly green (绿 lǜ), which the lǜ reading evokes nicely.
