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Carbon is the 15th most abundant element in the Earth’s crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. The atoms of carbon can bond together in different ways, termed allotropes of carbon. The best known are graphite, diamond, and amorphous carbon. 2 is found in carbon monoxide and transition metal carbonyl complexes. The allotropes of carbon include graphite, one of the softest known substances, and diamond, the hardest naturally occurring substance. Thus, irrespective of its allotropic form, carbon remains solid at higher temperatures than the highest-melting-point metals such as tungsten or rhenium. Carbon is the sixth element, with a ground-state electron configuration of 1s22s22p2, of which the four outer electrons are valence electrons.
Its first four ionisation energies, 1086. The electronegativity of carbon is 2. Although it forms an extraordinary variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. Carbon combines with some metals at high temperatures to form metallic carbides, such as the iron carbide cementite in steel and tungsten carbide, widely used as an abrasive and for making hard tips for cutting tools. Graphite is one of the softest materials known. Synthetic nanocrystalline diamond is the hardest material known. Graphite is a very good lubricant, displaying superlubricity.
Graphite is a conductor of electricity. Diamond is an excellent electrical insulator, and has the highest breakdown electric field of any known material. Graphite crystallizes in the hexagonal system. Diamond crystallizes in the cubic system. Carbon nanotubes are among the most anisotropic materials known.