A soft, heavy metal element that is highly toxic and was historically used in rat poisons but is now tightly controlled.
From Greek 'thallos' meaning 'green shoot' because thallium produces a bright green spectral line; discovered in 1861 and named for its characteristic color.
Thallium is so poisonous that it was once used as a murder weapon and appears in crime novels, but it's also used today in infrared detectors and specialized scientific instruments.
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