Thallium

Thallium sulfate is colourless, tasteless, water-soluble and lethal at about a gram, which made it an effective rodenticide and an effective poison; Agatha Christie used it in The Pale Horse in 1961, and her description of the symptoms has since been credited with helping doctors identify at least two real poisonings. The mechanism is that Tl⁺ has nearly the same ionic radius as K⁺, so cells take it up through potassium channels and it then disrupts everything potassium was doing — which is also the basis of thallium-201 cardiac perfusion imaging, where the tracer accumulates in well-perfused heart muscle. Crookes discovered it in 1861 by the bright green line in its spectrum, and named it from Greek thallos, a green shoot.