Properties of a Single Crystal

Smythe

In a mock exam I was asked for two properties associated with a single crystal.

For one answer I gave that they are hard.

I think another answer might have been something to do with crystal planes as this is what the teacher said when going over the paper again.

I was wondering what this might be? Are crystal planes the same thing as grain boundaries?

Thanks very much.

gerardduff

Smythe wrote: »

In a mock exam

I'm guessing you are at leaving cert level? So then I'm presuming your exam asked for a definition of a crystal i.e. a scientific definition, and then maybe asked to list some common properties of crystals.
Here's a definition and a few properties (in my own words so you'd better consult your textbook to get a definition an examiner would like to see .)

Definition.

Crystals are well ordered arrangements of atoms and/or molecules is space.

Properties.

Crystals usually occur in the solid phase. (liquids and gases are not usually considered crystal).
Crystals have a well defined defraction pattern. (Their emission spectra have well defined peaks)
Crystals have a well defined melting point (things that melt over a short range of temperatures (1-10 degrees C) are purer and have a crystal structure).

So you can see that if crystals are well ordered in space, like those things you see in a chemistry book with molecules linked together with rodlike things, then it's reasonable to 'slice through' the crystal and then consider what atoms and molecules you see lying in that plane.

The properties of a crystal are varied, and depend of the type of crystal you are talking about, some are insulators (pure silicon) others are conductors (most metals). Rather counter intuitively, hardness, which is a relative thing anyway, is not necessarily a property of a crystal. Examples are aluminium and iron.


With regard to your exam answer; properties themselves can be categorised, hardness is a mechanical property. You can look this up yourself as it is already off topic. Google the keywords hardness, stress, strain, young's modulus. Magnetism is another category and there are many more depending on what scale you are interested.

Smythe

Thanks very much gerardduff. That's very much appreciated!

Was just what I was looking for.