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Figure 1. Palladium as an element. Palladium (Pd) is a group-10 element (vertical columns) in the periodic table of elements. It has atomic number 46, resulting in 46 protons and 46 electrons. Its primary quantum number is 5, corresponding to its occurrence in the fifth horizontal row of the table. Virtually all properties of Pd, as an element and as a component of biomedical alloys, are attributable to its electron configuration (shown below the table). The outer shell orbitals of Pd (5s and 4d) contain ten electrons in total. Although the 5s orbitals are lower in energy than the 4d orbitals, and should therefore be filled first, the two sets of orbitals are close enough in energy that the 5s electrons are 'transferred' to the 4d orbitals to complete the outermost d-shell. This stable d-configuration contributes to the nobility of Pd and its ability to mix with other noble metals (see main text).
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Figure 2. Face-centered cubic lattice structure of palladium. Palladium forms solid solutions with several elements, including gold, silver and copper. These solid solutions are the basis for many of the biomedical alloys, and are generally desirable because of their low corrosion, ease in manufacturing, strength and ductility. The ability of elements to form solid solutions depends on the ability of the components of the solution (indicated by the two types of spheres above) to coexist in the metal lattice. Two elements must generally have similar atomic radii, electronegativities, valences and lattice structures to form a miscible series of solid solutions, such as palladium and gold or palladium and copper do (see also Table 1 ). In the formation of a solid solution, the second alloying elements will substitute for palladium at one or more of the atomic centers.
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Figure 3. Corrosion of Pd-Cu experimental binary alloys. A series of Pd-Cu binary alloys were fabricated into small disks that were exposed to cell-culture medium (63.5 mm2 /ml) for 72 h. The Cu (upper graph) or Pd (lower graph) release into the culture medium was measured by atomic absorption spectroscopy. The release of neither element was proportional to its atomic abundance in the alloys. Pd suppressed the release of Cu until it...