Atomic mass, symbol and atomic number Thallium is a naturally occurring element with the elemental symbol Tl and the atomic number 81. In the periodic table it stands with an atomic mass of 204.38 u in the 3rd main group. The chemical element discovered by William Crookes in 1861 is not radioactive and is in a solid state at room temperature.
Microorganisms (or microbes, as they are also called) are small organisms. Most microorganisms are harmless to humans and, in fact, many are helpful. They play fundamental roles in ecosystems everywhere on earth, forming the backbone of many food webs. People use them to make biofuels, medicines, and even foods.