氕 (radical 84, 气+1, 5 strokes, cangjie input 人弓中 (ONL), four-corner 80217, composition ⿹气丿)
Ideogrammic compound (會意): 气 (“gas”) + 丿 (“one”). Also a phono-semantic compound (形聲), where 丿 (piě) acts as the phonetic component. Compare 氘 (“deuterium”) and 氚 (“tritium”).
From 丿 (piě, “left falling stroke”) from the glyph, possibly influenced by New Latin protium. In Mandarin, it is pronounced in the first tone by analogy with 氘 (dāo, “deuterium”) and 氚 (chuān, “tritium”).