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can anyone pls suggest the reason why,CO2 and SiO2 are not isostructural at 300K???

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When comparing molecules and ions that are isoelectronic, only with the respect to their valance electrons, the expectation of isostructural behaviour may not hold. To take your situation: CO2 and SiO2 at room temperatur and pressure, CO2 is a linear molecule but SiO2 has an extended strcture containing silicon atoms in terahedral environments. An extended solid phase form of CO2 has been made at about 1900 K and 40 gigapascal pressure (If I remember right). This has a quartz-like structure, quartz being a 3-dimensional polymorph of SiO2. In the gas phase however we find SiO2 as linear triatomic molecules. A reason for this can be explained saying the vibrational modes may be thermally excited (in a classical interpretation one expresses this by stating that “the molecules will vibrate faster”), but they oscillate still around the recognizable geometry of the molecule.
does geometry of these molecules takes the major part in this case???
overall and simply why they do not exist isostructural at this temperature? is it b'coz of the structural change or any other???

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kk. Had your lectures about temperature, internal energy, molecular energy and vibrational modes? (perhaps morse potential aswell)
nw im going through the lectures of internal energy :)
Alrightie :) Anyway you know that the energy for the distance between atoms looks like this right?|dw:1356793711039:dw|
Well same idea aplies to angles between atoms - beside they are diffrence from atom to atom (in some cases)
fine explain more pls....
The energy levels of the two molecules are diffrence... CO2 is more stable in its linare form at 300 K (room tempture), while SiO2 is not.
Sorry my drawings suck..
is SiO2 having a constant graph?
nt abt the drawings it s about the matter we r discussing:)..... i can undestand :) looks gud anyway...
|dw:1356794123184:dw|It would (without being sure) maybe look like this:
Else I can suggest you read about vibrational modes of molecules, I sure belive I will becuase I'm a bit unsure (molecular geometry is not my best :P) :)
i hav no idea abt vibrational modes.... suggest any reading materials... if u can :)
What textbook do you use? (might be I have it)
open university books...
In that case I see if I can find some lecture notes here from the University of Copenhagen.
ya pls....
That are not in Danish :P
in English???
What I ment ;)
Bt u explained me some basic ideas... :) thanx alot 4 dat
Well I wish i could explain it better, mostly only work with the equations and then make conseqences from that.. Anyway, I can find some good reading in my open lectures, however else take a look at the wiki:
im on wiki nw thanx :):)
No prob, have a good day and if you are good with organic chemistry please see if you can answer my question :P
i tried bt couldnt... im jst a beginner:).... if i find anywhere... sure i'll let u knw... :)
Carbon is a lot smaller than silicon. Being that small, it can more readily involve the closer-in p valence orbitals to form pi bonds to neighboring oxygen atoms -- which gives you the CO2 molecule. Silicon, on the other hand, has a harder time involving the 3p orbitals in pi bonding to its neighbors, so it generally prefers to stick to sigma bonds. That means to satisfy its valence it forms bonds to four -- not two -- neighboring oxygens, which results in the extended diamondoid structure of silica at 300K. This is one of the illustrations of how the first element in each group often behaves nontrivially different from the others. One of the more significant is that the second row elements, being much smaller, form pi bonds much more readily than the elements in Period 3 and higher. You can see a similar effect in Group 6A: oxygen readily forms the double-bonded O2 molecule and exists as a gas at STP, while sulfur, just below it, instead prefers to form extended rings and chains of single bonds, and exists in the solid state at STP. Same with Group 5A: nitrogen forms the triply bonded N2, while phosphorus exists as the pyramidal P4 and is a solid at STP. No idea what molecular vibrations has to do with anything.
do u mean the different types bonding of these two plays the key role???
No, I mean the size of the atoms plays an important role in the types of bonding each prefers, and that, in turn, explains why they form two different types of oxides.
fine.... size might be factor... i didnt think abt the size...thanx alot :)
Carl-Farm, your way is way more specific, and also giveing a more actural reason, but the reason i used molecular vibrations is just a matter of approach, however your explination is more specific, what I wrote is only what I have experinced and choosed to see as energy that changes the molecule's 3-dimensional geometry.

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