-2
$\begingroup$

For example, let us take amorphous and crystalline quartz. While in crystalline quartz all bond angles are uniform, in amorphous form bond angles vary even though quartz has the same valency. Why is it so?

Structures of crystalline and amorphous quartz

$\endgroup$
  • 1
    $\begingroup$ One word: symmetry. Note that using all CAPS letters is inappropriate and is considered offensive in virtually any context; also use the punctuation marks and don't drop a wall of text without any of them! $\endgroup$ – andselisk Jun 9 at 14:11
  • 2
    $\begingroup$ I think you really want to ask why quartz can have these odd bond angles, right? If quartz glass had a periodic structure, it'd be crystalline. Contradiction in terms. $\endgroup$ – Karl Jun 9 at 15:48
  • $\begingroup$ I object. Amorphous quartz mostly has normal bond angles, just as well as crystalline. $\endgroup$ – Ivan Neretin Jun 9 at 17:07
  • $\begingroup$ Source of figure e.g. studiestoday.com/sites/default/files/rdsharma-solutions/… $\endgroup$ – Karsten Theis Jun 11 at 19:26
1
$\begingroup$

As a primer:

A requirement to qualify as crystalline, you need predictive patterns both in short, as well as in long distance range. In other words, there is a smallest structural pattern (unit cell) describing all atom's relative position well enough, which then is only replicated by translation. Which is the case in crystalline state (e.g., quartz, or sodium chloride), but not in the amorphous state (like glasses in general, not only quartz glass or glass in the windows).

However, it seems good to add some grains of salt:

  • The property "crystalline" is about order. It is not restricted to "solid state". A solid sample may be crystalline, but needn't be -- glasses are an example. On the other hand, a crystalline material needn't to be solid -- think about examples of liquid crystals seen in LCDs.
  • Mentioned "translation" may occur in all three directions in space. But putting an other unit cell just next to an other unit cell to build a crystal need not to occur this way, there equally are 2D (think about tiles in your bathroom) and 1D crystals, too.
  • There are cases outside this strict discern "crystalline or armorphous". There are polymers neither fitting into one, or the other category with quite some impact on the physical porperties (cf. for example this reference). An other exception to the simplification above are quasicrystals, which are ordered yet not periodic; and still crystalline.
$\endgroup$
  • 1
    $\begingroup$ Quartz can be amorphous or crystalline, and both in close proximity. So do polymers, nothing really special there imo. $\endgroup$ – Karl Jun 9 at 15:46
1
$\begingroup$

The variation in bond angles is much less than in the OP's diagram. Here is a bilayer of quartz observed under a scanning microscope:

enter image description here

Source: https://phys.org/news/2014-06-thin-silicate-atomic-glass.html

It shows a transition from crystalline (left) to amorphous (right). In the transition, the number of rings with six silicon atoms decreases in favor of some with seven or five, mostly.

The amorphous form has higher potential energy (some bond angle strain) and I guess higher entropy. Glasses are metastable, meaning that the crystalline form is thermodynamically favored, but the kinetics of reaching it are very slow.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.