Question

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

Answer 1

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.

Answer 2

does geometry of these molecules takes the major part in this case???

Answer 3

overall and simply why they do not exist isostructural at this temperature? is it b'coz of the structural change or any other???

Answer 4

uni...

Answer 5

kk.
Had your lectures about temperature, internal energy, molecular energy and vibrational modes? (perhaps morse potential aswell)

Answer 6

nw im going through the lectures of internal energy :)

Answer 7

Alrightie :)
Anyway you know that the energy for the distance between atoms looks like this right?