Metals - Bonding and Crystal Structure

Metallic Bonding Formed between atoms of metallic elements Good conductors of heat and electricity, lustrous, relatively high melting points, malleable and ductile. Examples; Na, Fe, Al, Au, Co

Structure of Metals Metals have low EN – they lose their valence electron(s) easily Become positively charged kernels. Metallic crystal structures are a frame of kernels. The electrons from each metallic atom are found in a common pool or “sea” and are free to move between all the kernels. They are called delocalized electrons.

Metallic Bonding - A Sea of Delocalized Electrons

Crystal Structures Metal kernals are viewed as hard spheres The packing pattern adopted depends on the size of the kernel and the number of e- s released into the sea. The metal bonding strength follows Coulomb’s Law - The smaller the kernel and a larger number of released e- s means a stronger bond.

Packing types Body Centered 8 nearest neighbors. 68% packing efficiency Alkali metals pack this way, most malleable, softest Close Packed -Hexagonal cubic -Face centered 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 kernels, the rest is filled by electrons. used by Be, Co, Mg, Zn, Sc, Ti, Cd & Zr Least malleable packing, hardest

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 More malleable than hexagonal, softer

Close Packing

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.

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.