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I am trying to understand malleability in metal as a result of the micro structure. I am focusing on the metals of antiquity (in order of decreased malleability): Gold, Silver, Copper, and Iron. While the atomic number decreases is it as simple as that? Or is the type of orbital more important? I think there is something about the d orbital, even if filled, that becomes part of the "delocalized sea" in the crystal structure, not just the regular valence electrons.

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    $\begingroup$ Could you cite the source of that ranking of metals by malleability? Thanks! $\endgroup$ – pentane Oct 31 '14 at 18:44
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    $\begingroup$ Mechanical properties are fairly complex, and not amenable to simple reasoning. Amongst other things, Au, Ag, and Cu are fcc crystals, while Fe is bcc. The different crystal structure results in very different dislocation behavior. And, if d orbitals were so important, than Ta, W, Re, Os, and Ir would be malleable, but alas they are not in the least. $\endgroup$ – Jon Custer Oct 31 '14 at 19:15
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As Jon Custer says Mechanical properties are fairly complex. When you bend a metal you cause dislocations in the atomic structure, which is normally uniform.

Here are some factors that affect malleability

The crystal structure: It is a large factor in dislocation motion.

Grain density: Dislocations cannot move past grain boundaries, so a higher grain density will make the metal harder.

Interstitials: Impurities in the metal (which every metal has) can make dislocations harder to move.

Temperature: The temperature that a metal is at can play a role in the crystal structure, and affects the inter-atomic bond strength. An extreme example of this is hydrogen "metal".

Atomic Size: The further apart atoms are from each other the weaker the force is, and the easier they move.

I suppose the orbitals do play a role in determining the lattice structure.

Most importantly though, as you mentioned, is the "delocalized electron sea". It is what allows dislocations in, an otherwise rigid, crystal structure to move. To anthropomorphize atoms; they can move around because they don't have to "worry" about maintaining the correct charge.

In ionic structures, such as NaCl, the atoms must remain next to an atom of the opposite charge. Thus enough energy to cause dislocation motion simply causes the like charged ions to be next to each other, and the crystal breaks along that plane. (This isn't perfectly true, but generally it is hard to cause dislocations in ionic materials.)

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    $\begingroup$ Is there a reason this is community wiki? $\endgroup$ – Pranav Hosangadi Dec 8 '14 at 0:14

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