Lecture 13 - Covalent bondingcan hold the solid together if...

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Lecture 13: Liquid crystals, and the transition to solids Properties of solids They have a definite volume, usually slightly smaller than the equivalent volume of the liquid (water is an exception), and a definite shape They have essentially no translational kinetic energy. They are held together by interactions at a well-defined distance. Energy absorbed by a solid below the melting point is used to break these interactions and increase vibrations, not to accelerate the molecules The interactions are often optimized for a highly regular arrangement of the subunit atoms or molecules, leading to crystallization . If a solid has no regular arrangement, it is amorphous. Interactions in solids Metallic bonding—holds the entire solid together, leads to metallic properties Ionic bonding—also holds the entire solid together except at high temperatures or in unusual liquids where ions are paired but shielded
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Unformatted text preview: Covalent bondingcan hold the solid together if the bonds can form a network. Examples are silicon oxide, boron nitride, silicon carbide, diamond Weak interactionshold the solid together if the covalent bonds lead to single molecule Lewis structures rather than networks. Examples are wax, naphthalene, and the sulfur eight-membered ring Borderline cases Hydrogen bonded compounds like ice and solid ammonia are somewhat molecular and somewhat network Solids can be partly crystalline and partly amorphous (polymers), or polycrystalline Solids can be partly regular and partly fluid: liquid crystals! Solids can have different interactions in different directions: graphite, mica As always, the borderline cases can be evaluated using the tradeoffs between H and TS contributions to free energy...
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