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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: jclough on May 03, 2009, 12:56:28 PM

Title: CsAu - ionic/metallic?
Post by: jclough on May 03, 2009, 12:56:28 PM

Hi everyone,

Why do caesium and gold form an ionic compound, rather than an alloy?

The only idea I have for explaining this is that being at the bottom of Group 1, electrons are much more easily taken from Cs' outer valence orbital (than say, K) and therefore there is a greater energetic likelihood that they will be localised in an ionic bond... perhaps you could say the enthalpy of lattice formation is greater (than for, KAu for example)... I'm not sure that argument really dispels the idea that CsAu could be an alloy.

I think at the root of my apprehension is that I don't really know what favours ionic over metallic bonding (and vice versa).

Thank you in advance for any insights to this problem!

Title: Re: CsAu - ionic/metallic?
Post by: ARGOS++ on May 03, 2009, 01:10:52 PM

Dear jclough;

Have you thought about the high electronegativity of Au and the difference?:
                 http://en.wikipedia.org/wiki/Ionic_bond 
                 http://en.wikipedia.org/wiki/Electronegativity#Electronegativities_of_the_elements

Good Luck!
                    ARGOS⁺⁺

Title: Re: CsAu - ionic/metallic?
Post by: Fleaker on May 03, 2009, 02:36:27 PM

Oh, but they do indeed make an alloy.

I can't really recommend you try mixing gold powder with cesium and heating though!

Title: Re: CsAu - ionic/metallic?
Post by: jclough on May 03, 2009, 06:07:20 PM

Hi Argos,

I knew that the electronegativity of gold was not too far off that of the halogens. What I am unsure over is how the small difference in the electronegativity between potassium and caesium could make such a big difference to the resulting bonding- i.e. that in a 1:1 ratio of the respective elements, KAu is an alloy and CsAu is an ionic compound (well, within the confines of this particular question...)

Thank you for your response!

Title: Re: CsAu - ionic/metallic?
Post by: ARGOS++ on May 03, 2009, 06:24:14 PM

Dear jclough;

I don’t know more then the abstract:
   http://www3.interscience.wiley.com/journal/112430908/abstract
   http://www.informaworld.com/smpp/content~content=a773260250~db=all
   http://books.google.com/books?id=hpWzxTnQH14C&pg=RA3-PA185&lpg=RA3-PA185&dq=cesium+gold+ionic&source=bl&ots=wS8JCUKajs&sig=ilIDgtt-N6jDuyC1rOIvr9mYKR4&hl=en&ei=khf-SezkKMiIsAbfnZXjBA&sa=X&oi=book_result&ct=result&resnum=8

Good Luck!
                    ARGOS⁺⁺