An alternate usage in metallurgy refers to metals which are found chemically uncombined in nature. Most protons remained uncombined as hydrogen nuclei. Consequently, platinum is often found chemically uncombined as native platinum. It never appears uncombined in nature and almost never stays uncombined for long. As a result, platinum is often found chemically uncombined as native platinum. Only a minority of elements are found uncombined as relatively pure minerals. Occasionally, however, metals are found in uncombined metallic forms, in varying degrees of purity. N. Dinitrogen forms about 78% of Earth's atmosphere, making it the most abundant uncombined element. Native copper is an uncombined form of copper that occurs as a natural mineral. It is extremely difficult to isolate from any compounds, let alone keep uncombined. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores.