Thallium (noun)

1. Thallium is a chemical element with the symbol Tl and atomic number 81. It is a soft, malleable and silvery-white metal that is highly toxic.
2. In medical use, thallium is used as a radioactive tracer for diagnostic purposes.

Origin:

Thallium is derived from the greek word "thallos," meaning "green shoot or twig." the element was named for its bright green spectral line which was discovered in 1861.

Examples:

1. Thallium is a highly toxic substance and can cause serious health problems if ingested.
2. The use of thallium as a pesticide was banned in the 1970s due to its toxicity.
3. Thallium salts are used in some electronic devices, including photodetectors and infrared detectors.
4. Thallium has been used as a murder weapon due to its ability to mimic symptoms of other illnesses.
5. The discovery of thallium was made by William Crookes, who observed its unique green spectral line.