Arsenic

Arsenic is a gray, brittle metalloid best known as a poison; for centuries it was a favorite of murderers because its compounds were tasteless and hard to detect. It occurs in many metal ores and is mostly recovered as a byproduct of smelting copper, gold and lead. It can also seep naturally into groundwater, a serious health problem in parts of South Asia.

In electronics, a little arsenic goes a long way. Paired with gallium, it forms gallium arsenide, the crystal that makes the infrared LED in the remote control. Implanted into doped silicon, each arsenic atom adds an extra electron, creating n-type regions in transistors.