4.8 The Structure of Solids

Solids have a definite shape and volume virtually incompressible, and do not flow readily Solids have a variety of properties: hardness, melting point, conductivity, mechanical behaviour These properties arise from the electrostatic forces in between the particles

There are different categories of solids according to the kinds of particles located at sites in the crystal lattice structure and the kinds of attractions that exist between the particles. Class of SubstanceElements that CombineExamples IonicMetal & Non-metalZnS (s), CaF (s), MolecularNon-metal(s)H 2 (s), I 2 (s), CCl 4 (s), CovalentMetalloids/carbonSiO 2 (s) quartz, diamond C (s), MetallicMetal(s)Fe (s), Mg (s), CuZn 3

Ionic Solids Hard and brittle (shatter into smaller crystals when stressed) –Why? When hammered or stressed, the crystal then literally self-destructs because of electrostatic repulsion High melting points Conduct electricity well when melted –Why? Properties reflect the strong attractive forces between ions of opposite charge as well as the repulsions that occur when ions of like charge are placed near each other

- Crystal lattice structure mmm… pass the salt please!

Metallic Crystals Conduct heat and electricity well –Why? valence electrons can freely move throughout the metal because of empty valence orbitals Have lustre –Why? Mobile valence electrons absorb and re-emit the energy from all wavelengths of visible and near- visible light Flexible –Why? nondirectional bonds mean that the planes of atoms can slide over each other while remaining bonded

Varies from hard to soft – Why? Depends on the strength of the positive nucleus and its attractiveness to electrons Melting point high to low –Why? Depends on the position of valence electrons in orbitals Continuous compact crystal structure –Why? electrons provide the “electrostatic glue” holding the atomic centers together –producing structures that are continuous and closely packed

Metal solids have positive ions at the lattice positions, which are surrounded by electrons in a cloud, that spreads throughout the entire solid (sea of electrons)

Molecular Crystals Low melting points -Why? low amount of kinetic energy need to break away from the solids form Soft –Why? because the particles in the solid experience relatively weak intermolecular attractions nonconductors of electricity in their pure form as well as in solution –Why? Their molecules are neutral and don’t allow for a charge to be carried

Molecular solids can be waxy solids (large hydrocarbons), giant polymers (such as plastics) and crystals Have lower levels of forces like dipole dipole, and hydrogen bonding Largest intermolecular force tends to be London dispersion forces Crystal lattice like ionic compounds, but the arrangement may be more complicated

Naphthalene- a molecular solid only attractions between the molecules are the London forces –Very volaitile “smelly”

Covalent Network Crystals Hard Very high melting points –Why? many covalently bonded atoms Do not conduct electricity –Why? because the electrons are bound too tightly to the bonds Insoluble Sometimes called network solids

3-D arrangement of atoms held together by strong, directional covalent bonds

Semiconductors A substance that conducts a slight electric current at room temperature but has increasing conductivity and higher temperatures Achieved by taking a metaloid that is a covalent crystal and “doping” it with another element

Homework Pg. 254 # 2-9