Left metal radical - side-form of 金, used for every metallic chemical element. 锌 is zinc, and like 铁 iron, 铜 copper, 银 silver, it picks up the metal radical to mark its place in the periodic table.
Right 辛 supplies the sound xīn with no shift - a tidy phonetic chosen when chemists coined a name for zinc. 辛 means 'bitter, pungent'; the meaning is unrelated to the metal, but the sound matched the German-derived 'Zink' perfectly.
