A chemical element, symbol Ag, atomic number 47, atomic mass 107.868. It is a gray-white, lustrous metal. Chemically it is one of the heavy metals and one of the noble metals; commercially it is a precious metal. Copper, silver, and gold make up group I of the periodic table of elements. Silver has been known as a metal since very ancient times; it was mentioned in the books of the Egyptian king Menes, in about 3600 B.C., who set its value at two-fifths that of gold. The chemical symbol Ag is derived from the Latin word for silver, argentum. Twenty-five isotopes of silver, including nine nuclear isomers, have been reported. Their atomic masses range from 102 to 117. Of the radioactive isotopes, the shortest half-life is that of ¹¹⁴Ag, 5 s, and the longest that of ^108mAg, about 5 years. Ordinary silver is made up of the isotopes of masses 107 (52% of natural silver) and 109 (48%).