|MadSci Network: Physics|
Hello "No name entered", For crystals held together by electrostatic forces between core ions (e.g. NaCl) the picture is this: | | | | | | | | | | | | - A - B - A - B - A - B - - A - B - A - B - A - B - | | | | | | | | | | | | - B - A - B - A - B - A - - B - A - B - A - B - A - - A - B - A - B - A - B - - B - A - B - A - B - A - | | | | | | | | | | | | - B - A - B - A - B - A - - A - B - A - B - A - B - | | | | | | | | | | | | The left side can fuse, because A and B (which attract) are next to each other. The right side can't, because of electro- static repulsion between A-A and B-B. To put together two crystal surfaces would thus involve a placement accuracy of the size of an atom (approx. 10e-10m). Nobody can do that! In metals the (positive) core ions are swimming in a sea of electrons. Yes, they are literally swimming - this is the reason why metals are malleable. Here, the placement is uncritical and perfectly flat (on an atomic scale) and clean surfaces would indeed fuse together. But there are no perfectly flat and clean surfaces in the real world. In fact, the parts of two surfaces which touch are usually negligible. It is, however, possible to prepare very flat and clean metal parts in a laboratory - and then you can feel the adhesion (although it is still possible to separate the parts). During welding, the metal is melted and completely fills the gaps - the parts are linked. Electrostatic crystals cannot be welded. I hope this answers your question. Greetings from Singapore, Frank Berauer
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