钅 is the left-side form of 金 and marks 钾 as a metal element. Chinese systematically uses 钅 + a phonetic for metal elements coined in the 19th-20th century: 铁 iron, 铜 copper, 银 silver, 钠 sodium, 钾 potassium.
甲 gives the sound jiǎ exactly - a clean phonetic borrowing for the chemistry name. Traditionally 甲 means "the first" or "shell/armour" but neither sense applies here; pure sound-matching for the periodic table.
