Thallium is a soft, grey post-transition metal represented by the symbol Tl. It resembles lead in appearance and is highly toxic in nature.

Thallium was once used in rat poisons and insecticides, but due to its extreme toxicity, such uses are now banned. Today, thallium is used in electronic devices, optical lenses, infrared detectors, and medical imaging equipment. Thallium compounds are also used in research and specialized industrial applications. Careful handling is essential due to its harmful effects on humans and the environment.