Concept 01 / 07Solid State
General Characteristics and Classification of Solids
Characteristic properties of the solid state, crystalline versus amorphous solids (long-range order, sharp melting point, anisotropy, cleavage), and classification of crystalline solids into molecular, ionic, metallic and covalent network solids by the nature of their binding forces.
Solids have definite mass, volume and shape; short, fixed intermolecular distances; strong intermolecular forces; and constituent particles that only oscillate about fixed positions.
Crystalline versus amorphous solids.
Crystalline solids have a regular, repeating three-dimensional arrangement of particles over long distances (long-range order). They have sharp melting points, a definite heat of fusion, and cleave along definite planes into smooth surfaces. They are anisotropic: properties such as refractive index and electrical conductivity differ along different directions, because the arrangement of particles differs in different directions. Examples: NaCl, quartz, metals, ice.
Amorphous solids have only short-range order, like a frozen liquid (pseudo solids or supercooled liquids). They soften over a range of temperature, have irregular cleavage, and are isotropic. Examples: glass, rubber, plastics, amorphous silica. Old glass window panes are slightly thicker at the bottom and some old glass objects become milky (partly crystalline) on long standing. Amorphous silicon is used in photovoltaic cells.
Quartz (crystalline ) melts sharply; quartz glass (amorphous ) softens gradually.
Classification of crystalline solids (see figure).
Molecular solids: molecules held by intermolecular forces.
Non-polar molecular: weak dispersion forces (, , , Ar, ); soft, very low melting points, insulators.
Polar molecular: dipole-dipole forces (HCl, , solid ); soft, low melting.
Hydrogen-bonded: ice; hard, low melting.
Ionic solids: cations and anions held by strong electrostatic (Coulombic) forces (NaCl, MgO, ZnS, ). Hard, brittle, high melting; insulators as solids because ions cannot move, but conductors when molten or dissolved.
Metallic solids: positive metal ions in a sea of delocalised electrons (Fe, Cu, Ag, Mg). Hardness and melting point vary widely; malleable and ductile; good conductors of heat and electricity; lustrous.
Covalent network (giant molecules): atoms joined by covalent bonds throughout the crystal (diamond, (quartz), SiC, AlN). Very hard, extremely high melting points, usually insulators. Graphite is an exception: soft (layers slide) and a conductor (delocalised electrons in layers).
Key Takeaways:
- Crystalline: long-range order, sharp m.p., anisotropic; amorphous: short-range order, softens over a range, isotropic.
- Molecular solids are soft and low melting; ionic solids are hard and brittle and conduct only when molten or dissolved.
- Metallic solids conduct in the solid state; network solids are hardest.
- Pitfall: glass is amorphous even though it looks solid and rigid; graphite is a network solid that conducts.
Brain-Teaser 1: Classify: (a) solid , (b) SiC, (c) , (d) brass, (e) ice, (f) graphite.
Solution: (a) non-polar molecular; (b) covalent network; (c) ionic; (d) metallic (an alloy); (e) hydrogen-bonded molecular; (f) covalent network (layered).
Brain-Teaser 2: Why is a crystalline solid anisotropic while an amorphous solid is isotropic?
Solution: In a crystal, the spacing and kinds of particles met along one direction differ from those along another, so direction-dependent properties (conductivity, refractive index, mechanical strength) differ. In an amorphous solid, the arrangement is random on average in all directions, so the properties are the same in every direction.
Brain-Teaser 3: Ionic solids conduct electricity when molten but not in the solid state, whereas metals conduct in both. Explain.
Solution: In an ionic crystal the charge carriers are ions, fixed at lattice sites; they become mobile only in the melt or in solution. In a metal the carriers are delocalised electrons, which are free to move in the solid and in the liquid.
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