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Softness of Transition Metals Au, Ag, and Pd

by HeavyMetal
Tags: metals, softness, transition
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HeavyMetal
#1
Jun18-14, 01:18 AM
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Is there a direct reason that gold, silver, and palladium are so much softer than the rest of the transition metals? Gold and silver are especially soft. I could not think of any reason myself, or in a quick search of the internet. Everything that I've come up with I've shot down.

Thanks in advance :)
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DrDu
#2
Jun18-14, 04:43 AM
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In Copper, Gold, Silver, but also in the Alkali metals, which are all very soft, there is only metallic bonding due to the valence s electrons forming a Fermi fluid. This kind of bonding shows no directional character so that a motion of the ionic cores along some glide planes is easy. In hard and brittle metals like W, Mo, V, Ti and the like, there exist in addition strong directed bonds due to the overlap of d-orbitals. A deformation will have to break these bonds so that there is much more resistance to deformation.
HeavyMetal
#3
Jun18-14, 08:22 AM
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The odd thing is that, according to the Brinell hardness of these metals, Pd, Au, and Ag are extraordinarily soft. Au, Ag, and Pd, have values of 25, 24.5, and 37.3 MPa respectively. The next hardest transition metal is manganese with a value of 196 MPa, which is ~8x harder than gold and silver. And even manganese is very soft! Copper is NOT in this list, having a modest value of 874 MPa.

There is something unique about these three metals, and I just can't figure it out!

HeavyMetal
#4
Jun18-14, 10:20 AM
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Softness of Transition Metals Au, Ag, and Pd

I'm going to have to go to a better source, I'm starting to find conflicting numbers.
HeavyMetal
#5
Jun18-14, 06:30 PM
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I'm still finding that Ag and Au are pretty much the softest transition metals. I'm thinking relativistic effects are dominating, causing the s-orbitals to shrink, and allowing for the d-orbitals to be less tightly held. What do you guys have to say?
Astronuc
#6
Jun18-14, 10:25 PM
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The hardness/softness is somewhat coincident with melting temperature.

http://www.webelements.com/gold/physics.html

It relates to the filling of the d-orbitals, which are nearly complete for Cu, Ag, Au.

I believe these are fcc metals, whereas the harder/stronger metals, like W, Ta are bcc.
HeavyMetal
#7
Jun18-14, 11:37 PM
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Yeah, I guess it makes sense that if they're FCC, they'd shear easier. I was thinking that since Hg has one more 6s electron than Au, it could stabilize the nuclear charge better as well as providing for more contraction due to relativistic effects. This would then allow the remaining 10 d-electrons to be held even looser than in Au, explaining why it is liquid at room temperature.
DrDu
#8
Jun19-14, 03:39 AM
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P: 3,596
Yes, these relativistic effects are also important.


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