Chlorine is the chemical element with atomic number 17 and atomic mass 35.5. It is a yellow-green gas at room temperature, and forms diatomic molecules. It is a poison and was used in World War One as the first chemical weapon. Gas was released into the trenches which burned the skin and corrupted the lungs of the soldiers, leading to heavy casualties. Its chemical formula is Cl2. Chlorine is a reasonably reactive element, reacting with most other elements except the noble gases (helium, neon, argon, xenon, radon). In nature, chlorine has two isotopes: chlorine-35 and chlorine-37, which occur in a proportion of 75% and 25% respectively.