Arizona radiogenic helium dating laboratory
During the formation of the Solar System, there were probably many more (planetesimals), but they have all merged with or been destroyed by the four remaining worlds in the solar nebula.
The Terrestrial planets all have roughly the same structure—a central metallic core, mostly iron, with a surrounding silicate mantle.
However, because of the tiny amounts found in nature, uranium needs to undergo enrichment so that enough uranium-235 is present.
Its density is about 70% higher than that of lead, and slightly lower than that of gold or tungsten.
It occurs naturally in low concentrations of a few parts per million in soil, rock and water, and is commercially extracted from uranium-bearing minerals such as uraninite. Many contemporary uses of uranium exploit its unique nuclear properties.
Most nitrogen atoms have seven protons and seven neutrons, so their atomic mass is 7 7 = 14.
The fact that it has seven protons is what makes it nitrogen.