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Solutions to Q&S related to electronic structure of materials lecture . 1 a) Si+2Cl2=SiCl4 1 b) The geometric arrangement in SiCl4 consist of Si atom in a center of a tetrahedron and Cl atoms occupying the four corners of the tetrahedron. 1 c) Since the bonding geometry described in 1b) is symmetric it implies that the molecule has no permanent dipole moment because the centers of positive and negative charges coincide. Intermolecular forces between SiCl4 are thus expected to be very week (van der Waals forces). At room temperature the material is expected to be in a gas or liquid phase but not in a solid phase. 2 a) 2 b) Visible light contains radiation in the wavelength range 4000Å (violet)7000Å (deep red). A material that appears red in transmission light absorbs all the wavelengths of the visible light in the range 4000-6500Å and transmits wavelengths >6500Å. This material must belong to semiconducting class of materials since photons with energies of red radiation can excite electrons from valence to conduction band. Assuming that optical absorption starts at =6500Å we get the following value for the bandgap of the material: Eg=hc/ I defined in my lectures semiconductors as materials with a bandgap below 1.5 eV. However I also pointed that this value is a rather “diffuse” one definitely not a sharp one. Thus a bandgap of 1.9 eV makes a material a semiconductor.