Chemical element or an electrophile; similarly "Nu" denotes a nucleophile. "L" is used to represent a general ligand in inorganic and organometallic chemistry. "M" is also often used in place of a general metal. At least two additional, two-letter generic chemical symbols are also in informal usage, "Ln" for any lanthanide element and "An" for any actinide element. "Rg" was formerly used for any rare gas element, but the group of rare gases has now been renamed noble gases and the symbol "Rg" has now been assigned to the element roentgenium. Isotopes are distinguished by the atomic mass number (total protons and
Cobalt zinc oxide), and cobalt violet (cobalt phosphate) are used as artist's pigments because of their superior chromatic stability. Aureolin (cobalt yellow) is now largely replaced by more lightfast yellow pigments. Cobalt-60 (Co-60 or Co) is useful as a gamma-ray source because they can be produced in predictable quantity and high activity by bombarding cobalt with neutrons. It produces gamma rays with energies of 1.17 and 1.33 MeV. Cobalt is used in external beam radiotherapy, sterilization of medical supplies and medical waste, radiation treatment of foods for sterilization (cold pasteurization), industrial radiography (e.g. weld integrity radiographs), density measurements (e.g. concrete density