Thallium

Thallium was discovered in 1861 by William Crookes, who noticed a bright green line in the spectrum of selenium-containing sulfuric acid residues. He named it from the Greek 'thallos,' meaning green shoot or twig, for its distinctive green spectral line. Thallium is a soft, malleable metal that looks like tin but tarnishes quickly to a bluish-gray.

Thallium earned a dark reputation as the 'poisoner's poison' because thallium salts are colorless, odorless, tasteless, and lethal in small doses. Agatha Christie even featured thallium poisoning in her mystery novel 'The Pale Horse.' On the positive side, thallium is used in some types of glass for infrared optical equipment, in semiconductor devices, and thallium-201 is used in heart imaging scans to detect blocked arteries.

Thallium's symbol 'Tl' is a distinctive two-letter element in Spellement. While 'tl' is rare in English words, partial emphasis allows spellers to use 't' or 'l' individually — both very common consonants that appear in a huge number of English words, offering strong word-building flexibility.