arsenic
砷 centers on the chemical element arsenic, a brittle metalloid often found in minerals and used in alloys and semiconductors.
砷 is a modern coined character for arsenic, combining 石 (stone/mineral) with 申 as a phonetic component. It was created to represent the element in Chinese and later adopted into Japanese.
The left side 石 means stone or mineral, and the right side 申 looks like a stretched-out grid. Picture a brittle gray mineral (arsenic) being extracted from stone.
For シン, imagine a shiny, silvery arsenic crystal: shiny -> シン, and the stone 石 on the left reminds you it's a mineral.
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