Crystal Structures Metal kernals are viewed as hard spheres. • The packing pattern adopted provides the greatest energy stability. Ions in salts are also hard spheres • The packing pattern is the same as metals, now the larger ion. • The smaller ion is placed in the “holes” of the structure instead of electrons.
Unit Cells • Crystals consist of repeating units which may be atoms, ions or molecules. • The space lattice is the pattern formed by the points that represent these repeating structural units.
Packing types Each Metal kernal has: Body Centered • 8 nearest neighbors. • 68% packing efficiency • Alkali metals pack this way, most malleable Close Packed - Face centered cubic - Hexagonal cubic • 12 nearest neighbors.
Hexagonal Close Packing • Alternating planes of hcp spheres • Those in the 3rd plane pack directly above those in the 1st plane – ABA pattern • 74% of the space is filled by metal kernals, the rest is filled by electrons. • used by Be, Co, Mg, Zn, Sc, Ti, Cd & Zr • Least malleable packing
Face Centered Packing • Identical to hexagonal packing, EXCEPT the 3rd spheres are in the holes that were not used to form the 2nd plane • The 4th plane is above the 1st plane – ABCA • Used for Ag, Al, Au, Ca, Cu, NI, Pb and Pt • Less malleable than face centered cubic
Alloys Alloys are solid solutions of metals. They are usually prepared by mixing molten components. They may be homogeneous, with a uniform distribution, or occur in a fixed ratio, as in a compound with a specific internal structure.
Substitutional Alloys Substitutional alloys have a structure in which sites of the solvent metal are occupied by solute metal atoms. An example is brass, an alloy of zinc and copper.
Substitutional Alloys These alloys may form if: 1. The atomic radii of the two metals are within 15% if each other. 2. The unit cells of the pure metals are the same. 3. The electropositive nature of the metals is similar (to prevent a redox reaction).
Interstitial Alloys Interstitial alloys are solid solutions in which the solute atoms occupy holes (interstices) within the solvent metal structure. An example is steel, an alloy of iron and carbon.