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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: kkjc2 on November 17, 2005, 02:34:41 PM

Title: Why is Copper Red? and Caesium golden?
Post by: kkjc2 on November 17, 2005, 02:34:41 PM
Hi.
I am just wondering about the theoretical basis of the red colour of copper metal.

I have learnt that the yellow colour of gold (Au) is due to significant relativistic effect and resulting in contracted 5d and 6s shells and the transition from 6s --> 5d now falls within the visible region. (pls correct me if I am wrong or inaccurate)

This only affects heavy elements, so copper cannot be red due to this same reason.
Does it have a transition that absorb blue/green region? which one?

Also, it is interesting that caesium metal (Cs) appears as a golden coloured metal, does anyone know whether this golden colour is due to a very minor oxidation of the metal or whether there is a physical basis for this colour?

Very interested to learn more, Many thanks for any replies.
Title: Re:Why is Copper Red? and Caesium golden?
Post by: Juan R. on November 26, 2005, 08:59:14 AM
Quote from: kkjc2 on November 17, 2005, 02:34:41 PM
Hi.
I am just wondering about the theoretical basis of the red colour of copper metal.

I have learnt that the yellow colour of gold (Au) is due to significant relativistic effect and resulting in contracted 5d and 6s shells and the transition from 6s --> 5d now falls within the visible region. (pls correct me if I am wrong or inaccurate)

This only affects heavy elements, so copper cannot be red due to this same reason.
Does it have a transition that absorb blue/green region? which one?

Also, it is interesting that caesium metal (Cs) appears as a golden coloured metal, does anyone know whether this golden colour is due to a very minor oxidation of the metal or whether there is a physical basis for this colour?

Very interested to learn more, Many thanks for any replies.

About gold, yes you are correct, relativistic effects reduce the transition 5d-6s.

However, is not Cu-4s transition already of the order of 2-3 eV? I have not checked this!

I say because i am seeing now that dissociation energies for dimers are approx. (eV)

Cu2: 2

Ag2: 1.65

Au2: 2.3

Relativistics effects for 6s1 increase the energy breaking the nonrelativistic tendency 2 -> 1.6 -> 1.

For trimers, Au3 and Cu3 are still more similar with Ag rather different.

I think that Cu color may be normal and the only Au color is abnormal due to relativistic effects.

I continue searching information  ;)
Title: Re:Why is Copper Red? and Caesium golden?
Post by: Juan R. on November 26, 2005, 09:32:55 AM
Quote from: Juan R. on November 26, 2005, 08:59:14 AM
I continue searching information  ;)

I find this graphic

(http://www.minsocam.org/MSA/collectors_corner/arc/img/NassauF7.gif)

on http://www.minsocam.org/MSA/collectors_corner/arc/color.htm

I think that above explanation of Red color for Cu may be pausible. Cu is Red because its band gap may be on 1.8-2.0 eV. Ag is 3.5 eV and therefore is already colorless and Au would be > 4.0 eV and therefore colorless but due to relativisitc effects on 6s electron, the band gap reduces to 2.4 eV and color is yellow.
Title: Re:Why is Copper Red? and Caesium golden?
Post by: kkjc2 on December 03, 2005, 05:24:53 PM
Thank you very much for your time. I agree that your reasoning is very plausible.
Copper is naturally red for the 4s--> 3d transition.