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Essential Elements

22 23 carbon SIlIcon organic and virtual thinking materials Twentythree percent of you is carbon. The sixth element underpins organic chemistry, the fabric of life, from DNA and proteins in our cells to once living stuff, plastics and fossil fuels. Coming in a dazzling array of molecules, carbon is neither electropositive or negative. A nonmetal, it combines with many other elements and also extensively with itself, creating long chains and rings see page 55. Multiple bonds smear electrons between atoms to give double and triple bonds. Carbon arranges as several different allotropes. In diamond crystals every atom bonds to four others in a tetrahedral grid top right whereas in graphite, a soft crystalline solid found in charcoal and pencils, flat planes of carbon rings slide easily over each other top left. Each atom here joins to three others, the bonds enabling it to conduct electricity. Other carbon allotropes include a series of spherical buckyball molecules, the buckminsterfullerenes, and intruiging nanotubes that self assemble in the right conditions. Directly beneath carbon in the periodic table is silicon, a metalloid semiconductor. Carbon life is mirrored by silicon logic in the buzzing mazes of electronic microchips which consist of purified silicon crystals, doped with elements like gallium or arsenic to alter their conducting properties. Stable silicon compounds cover over a quarter of the earth as rocks and sand, and clays, silicates with unusual lifemimicking chemistries, perhaps even led to biological evolution.
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