Technetium (pronounce /tɛkˈniːʃɪəm/) is the lightest chemical element with no stable isotope. It is a synthetic element. It has atomic number 43 and is given the symbol Tc. The chemical properties of this silvery grey, crystalline transition metal are intermediate between rhenium and manganese. Its short-lived gamma-emitting nuclear isomer 99mTc (technetium-99m) is used in nuclear medicine for a wide variety of diagnostic tests. 99Tc is used as a gamma ray-free source of beta particles. The pertechnetate ion (TcO4-) has been suggested as a strong anodic corrosion inhibitor for mild steel in closed cooling systems.