1
$\begingroup$

Today my teacher taught about amorphous and crystalline substances. Crystalline substances are said to have different values for physical properties (like speed of light, electrical resistance, etc) in different directions. Crystalline substances are said to have orderly arranged constituents, however this orderly arrangement is not said to be same in different directions. Thus, they are said to show different values for physical properties in different directions.
Amorphous are said to be isotropic in nature, i.e they have same value for all the physical properties in different directions. I was stuck here, amorphous substances have disordered arrangement of constituents. So, they should strictly have different values in different directions. Is it we speak about average values of physical properties in case of amorphous substances? or is it that, they strictly have same values for the physical properties in all directions?

$\endgroup$
1
$\begingroup$

Yes, you’re exactly right that we are talking about an average. If we could somehow look at a small enough portion of an amorphous solid, we would see differences between the properties in different directions. Indeed, a small enough portion of, say, amorphous ice is a single water molecule, which is certainly not isotropic: we can talk about the direction perpendicular to the molecular plane or whatever.

But when we talk about real, physical materials, as usual in chemistry, we are talking about Avogadro’s number (plus or minus a few orders of magnitude) of molecules. So taking the spatial average of an amorphous structure makes perfect sense, and the law of large numbers means that the tiny differences between properties in different directions will be utterly undetectable.

(As an aside, it’s worth noting that the perfect crystalline structure we imagine is also an average over every unit cell in the material – it’s just that because each unit cell is oriented in more or less the same way, different directions are not “mixed” as part of this average, so crystals can have anisotropic properties.)

$\endgroup$
0
$\begingroup$

There is probably some misunderstanding here... As an macroscopic example, take a book. And then split it in half. There is only one way you can do it with small effort. That's anisotropy. Then run the book through paper shredder and you obtain isotropic material (http://www.compatibletoner.org/wp-content/uploads/2009/09/shredded-papers.jpg). You can split it easily in half in all directions and also other properties are independent of the direction (compared to the book).

$\endgroup$

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.