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How come Au does not react well like Cu and Ag?

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  1. May 11, 2015 #1
    Au is on the same group like Cu and Ag, they're all good conductors of heat and electricity due to their outer s orbit single electron: 4s1, 5s1 and 6s1 respectively. Cu and Ag often give up this last electron in chemical reactions to become +1 ions and they are active chemicals.
    Yet Au, albeit has similar atomic structure and similar s orbit outer electron, does not tend to do well in chemistry. How's that?
     
  2. jcsd
  3. May 11, 2015 #2

    Borek

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    They are not active, both are relatively noble. Silver is definitely more difficult to oxidize that copper, and gold is even more difficult to oxidize than silver. All three are between rare metallic elements that occur in the native form.
     
  4. May 11, 2015 #3
    Yeah but we have AgNO3, AgCl, AgNO3, CuCl2... to name a few. But Au is like a "noble solid". It doesn't even react to many strong acids. How come it has an electron at 6s1 to make it so good at conducting heat and electricity yet that electron is like glued to nucleus when it comes to a chemical reaction?
     
  5. May 11, 2015 #4

    Borek

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    You won't get better arguments than handwavy ones. It is all in a combination of ionization energies and interactions with other elements/substances, but it is hard to pinpoint a single reason.
     
  6. May 11, 2015 #5

    James Pelezo

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    Copper, Silver and Gold are frequently referred to as 'The Precious Metals Group'. Their reactive nature can be experimentally related to their reduction potentials. That is, how easily is the element reduced relative to a Standard Hydrogen Electrode. The metal of interest is used as an electrode ( Cathode to be specific ) connected to the standard hydrogen electrode ( Anode ) in the case of Cu, Ag & Au in a Voltaic Cell configuration. The voltage generated by the cell is called the Reduction Potential. The greater positive Reduction Potential, the more difficult to reduce and hence is classified as less reactive. Using the most common ionic forms, the Eo(Std Reduction Potential) [2H+/H2(g)] = 0.00 volts, [Cu+2/Cuo(s)] = +0.34 volts, [Ag+/Auo(s) = +0.80 volts and [Au+3/Auo(s)] = +1.52 volts. The higher positive reduction potential indicates the greater difficulty in reducing the ion to basic standard state and hence a decreased reactive nature. Structural arguments using electron configurations are also used, but are less convincing than actual experimental conclusions. I would suggest reviewing the chemistry of Galvanic/Voltaic Cells to better understand the character and reactive nature of metals as related to reduction potentials generated using a standard hydrogen electrode. Then examine their electron configurations in reduced and oxidized form to see what orbitals and electrons are involved as the metal undergoes transition from ionic form to reduced basic standard state.
     
  7. May 12, 2015 #6
    Thank you .
     
  8. May 12, 2015 #7

    DrDu

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    The reduced reactivity of gold as compared to silver has also to do with relativistic effects. Due to the high charge of the gold nucleus, the electrons near the nucleus reach velocities near the speed of light. This leads to a shrinking of the s-orbitals and the s orbitals becoming energetically stabilized.
     
  9. May 12, 2015 #8

    DrDu

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  10. May 12, 2015 #9
    Woavv, that's a golden piece of information, thank you.
     
  11. May 12, 2015 #10

    epenguin

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    Sorry for the overlap with Dr. Du but I had been chasing that point up yesterday



    I read that gold's exceptionality and non-reactivity is due to a relativistic effect! (Which also accounts for its colour).


    Special relativity is also responsible for gold's resistance to tarnishing and other chemical reactions. Chemistry is mostly concerned with the electrons in the outermost orbitals. With a single 6s electron, you might expect gold to be highly reactive; after all, cæsium has the same 6s1 outer shell, and it is the most alkaline of natural elements: it explodes if dropped in water, and even reacts with ice. Gold's 6s orbital, however, is relativistically contracted toward the nucleus, and its electron has a high probability to be among the electrons of the filled inner shells. This, along with the stronger electrostatic attraction of the 79 protons in the nucleus, reduce the “atomic radius” of gold to 135 picometres compared to 260 picometres for cæsium with its 55 protons and electrons—the gold atom is almost 50% heavier, yet only a little over half the size of cæsium. Only the most reactive substances can tug gold's 6s1 electron out from where it's hiding among the others, and hence not only the colour of gold, but its immunity from tarnishing and corrosion are consequences of special relativity.
    https://www.fourmilab.ch/documents/golden_glow/

    (I also mentioned some weeks ago that the fact that the lead battery works is also now explained by relativity - though this is harder to understand because it involves energy levels in compounds. https://www.physicsforums.com/threa...ipitation-of-unknown-ion.790077/#post-4980286)

    Despite the above gold does have some weird and wonderful chemistry that finds avant-garde applications in catalysis and in nanotechnology and it is a proving ground for supercomputational chemistry.

    I was particularly struck by a predicted buckminsterfullerene gold molecule and by the existence of a Au- ion.

    http://www.insp.upmc.fr/webornano/ressources/2009/pdf_dijon/02_Pyykko.pdf
    http://www.sciencedirect.com/science/article/pii/S129325580500230X

    Maybe best not to go far into this, you might never get out again!
     
    Last edited: May 12, 2015
  12. May 13, 2015 #11
    this is what I was looking for! thank you.
     
  13. May 13, 2015 #12

    DrDu

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    This is comparing apples and pears. Also rubidium is much more reactive than silver and potassium more than copper. So the difference in reactivity between cesium and gold isn't mainly due to relativistic effects. Rather, the smaller size of gold as compared to cesium is mainly due to the filling of the d shell. In fact, also the golden colour of cesium seems to be due to relativistic effects.
     
  14. May 13, 2015 #13

    Borek

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    I must say I always find attributing the changes in the size/reactivity/electronegativity/whatever to some specific thing ("shielding", "relativistic effects", "ionization energy", you name it) quite unrealistic. Rarely it is a single thing that is responsible, and often deciding in what part the change depends of a single thing is quite handwavy. Most often we trick ourselves into thinking "ah, so it is because of relativistic effects, now I know". You know nothing, John Snow.
     
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