Thallium is a metal — one of the chemical elements, written Tl on the periodic table. It is soft, so soft that you could cut it with a knife, and it is a blue-white or grey colour when it is fresh, though the surface goes dull quickly in air. Its most important quality, and the one people remember, is that it is very poisonous. A small amount can kill, and it has no taste and no smell, which is a dangerous combination. Because of that, it used to be sold as rat poison and insect killer, and many countries have now banned or restricted those uses. It has other jobs too: it goes into some electronic parts that react to light, into certain special glass for infrared equipment, and a form of it is used in hospitals to take pictures of how blood moves through the heart. You will also meet the word in crime stories, real and invented, because thallium poisoning is slow and hard to spot — the person loses their hair and their nerves hurt — so it was once called "the poisoner's poison." It is a technical word: you will see it in science class, in news reports about poisoning, and in hospital talk, not in everyday conversation.

鉈（化學元素，符號 Tl；台灣讀作ㄊㄚ）