Air/gas radical wrapping the upper-right — pictures rising vapor or breath curling upward. Modern science chose this radical to mark the family of gas elements: 氢 (hydrogen), 氧 (oxygen), 氮 (nitrogen), 氯 (chlorine), 氦 (helium). Spotting 气 immediately tells you 'this is a gas.'
羊 (yáng) supplies the sound, lightly retoned to yǎng. The choice of 羊 is purely sonic and dates from the late-Qing translation of Western chemistry (氧 was coined for 'oxygen' around 1900). Same phonetic threads 洋, 样, 养, 痒.
