In diamond, carbon is hybridized and has a tetrahedral molecular geometry. In graphite, carbon is hybridized and has a trigonal planar geometry. The overlap of the orbitals allow for the delocalization of the electrons, forming the strong carbon planar structure in graphite. Elemental silicon also has a diamond structure, meaning the silicon is also hybridized and has a tetrahedral molecular geometry. However, it doesn't have an allotrope in which silicon is hybridized and has a trigonal planar geometry. It is due to the fact that silicon doesn't form strong bonds with each other. This can be attributed to the atomic size of silicon. Since silicon has a bigger atomic size than carbon, the overlap of the orbitals are smaller, and thus the resulting bonds are weaker.
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