See slides 5 and 6 of the attached https://www1.columbia.edu/sec/itc/ee/test2/pdf files/silicon basics.pdf for a good description of calculating the density of silicon based on its structure.

For silica (quartz) the chemical formula of the basic structural unit is Si3O6 i.e., it comprises of 3 x SiO2 so it’s mass:

= ((3 x 28.085) + (6 x 15.999)) x (1.66 x 10^-24)

= (84.255 + 95.994) x 1.66 x 10^-24

= 2.9921 x 10^-22 g

And the volume of the unit cell = 112.28 Angstrom3 = 112.28 x 10^-24 cm3

Therefore:

As Density = weight/volume = (2.195 x 10^-22)/ (1.1228 x 10^-22)

= 2.665 g/cm-3

See slides 5 and 6 of the attached https://www1.columbia.edu/sec/itc/ee/test2/pdf files/silicon basics.pdf for a good description of calculating the density of silicon based on its structure.

For silica (quartz) the chemical formula of the basic structural unit is Si3O6 i.e., it comprises of 3 x SiO2 so it’s mass:

= ((3 x 28.085) + (6 x 15.999)) x (1.66 x 10^-24)

= (84.255 + 95.994) x 1.66 x 10^-24

= 2.9921 x 10^-22 g

And the volume of the unit cell = 112.28 Angstrom3 = 112.28 x 10^-24 cm3

Therefore:

As Density = weight/volume = (2.195 x 10^-22)/ (1.1228 x 10^-22)

= 2.665 g/cm-3

Dear Dr. Paul,  According to the tabulated value of  Si and  SiO2  in Google, one has:

Si:  density  2.328 g/cm3 , Melting Temp:  1415C  and Diamond Structure,

SiO2  density  2.2  g/cm3Melting Temp:about 1600 C  and Amorphous.

Best Regards

Note:  The General Properties of Si, Ge, SiGe, SiO2 and Si3N4

June 2002

Virginia Semiconductor

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Fused Silica (amorphous) has a denisty of 2.2 g/cm3 Quartz (crystalline) has a density of 2.65 g/cm3 see http://www.azom.com/article.aspx?ArticleID=1114

The density of materials is related to their Molar Volume (how the atoms are packed) which tends to be smaller in crystalline materials because of their geometric packing see http://www.virginia.edu/bohr/mse209/chapter3.htm

Dear Dr. Crowley,  As far as I remember   all of the metals and alloys with an exception of Be  after melting they show increase   in the specific volume.  Even though their coordination don' t change very much according to their atomic distribution functions  obtained by  XRD.  I think this is due to fact that there is substantial increase in the multiple- vacancy (or di-vacancies etc.,)) concentrations, which is some times  called 'relaxator'  that  appears first in the grain boundary regions, which melts much lower temperature then the bulk.  This may be seen as a hook on the straight line in the vicinity of the melting temperature if you  plot diffusivity versus inverse homologous temperature in semi-logarithmic scale. Fractional specific volume increase between  amorphous and crystalline SiO2 is about 20%, according to your reported values.

The regrowth  kinetics of epitaxial amorphous Si thin layer on Si substrate  under the electrostatic, and elastostatic forces was studied in  one our publication.(Please see the attached file.

Best Regards 

Density is mass/volume, so the thought here is that the mass of Si > O therefore mixtures of Si and O should be less dense then just Si. That idea focuses on the component masses but completely neglects the molar volumes i.e. neglects the entire concept of the solid state - that solids form specific packing motifs in order to pack efficiently.

The "answer" comes from ice. There are about 14 different phases of ice, all with different structures, and different densities and different packing motifs. Some are dense, some are not and yet all are "made" from water. When I want to learn more about the solid state I go to A.F. Wells (Structural Inorganic Chemistry and the associated works on nets/polyhedra), I've never found anyone with a deeper, more insightful yet clear and approachable knowledge of the subject.

Dr. Gordon-Whlie  is definitely on the right tract. The answer is hidden in the definition of density: Standard  definition:   d=  weight /volume.    This definition tells you that if you have two different material there is no mathematical connection between their densities even though you have knowledge about their chemical content.  You can go one more step and claim that same chemical species may have different densities if their crystal structure and/or defects contents would be different. 

I am sorry to say that this question is not well formulated.  How can you say Oxygen is lighter then SiO2 . You may say molecular weight of Oxygen is less than the molecular weight of SiO2.   But you can't say anything about their molar volume, which depends upon the physical state of species as Scott clearly pointed out.