Uranium (pronounce /jʊˈreɪniəm/) is a silver-gray metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and electrons, 6 of them valence electrons. It can have between 141 and 146 neutrons, with 146 (U-238) and 143 in its most common isotopes. Uranium has the highest atomic weight of the naturally occurring elements. Uranium is approximately 70% more dense than lead and is weakly radioactive. It occurs naturally in low concentrations (a few parts per million) in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite (see uranium mining).