[nwkegan]

Hello!

I start classes in spring, so I picked up "General Chemistry" by Linus Pauling in an effort to get a head start (and sate my thirst for knowledge.) I have been reading through and have picked up a few questions along the way (with more likely to come) that I hope one or two of you would not mind answering.


In chapter 2, Linus writes, "It is the regularity of arrangement of the atoms in a crystal that gives to the crystal its characteristic properties, in particular the property of growing in the form of polyhedra ...These faces lie at angles to one another that have definite characteristic values, the same for all specimens of the same substance... these faces are always orthogonal to one another." However, he THEN writes, "Native copper, found in deposits of copper ore, often is in the form of crystals with cubic and octahedral faces."

Why often? It seems contradictory to me that, in one place, he mentions all specimens of the same substance exhibit the same characteristics, and then later mentions that they are OFTEN found in this manner.

He also says that if a force is exerted upon a substance that its atoms can be compressed. The previously mentioned conditions (constant form, etc) all go out the window when this happens, correct? If so, is that how the preceding contradiction is explained?

Thanks for any help.

[cliverlong]

To be honest, I don't fully understand your questions but I will have a go based on my interpretation.

1. I am not familiar with the Pauling book and don't have a copy to hand.

2. Study of crystal structures requires an investigation and understanding of the structure on a macroscopic (human) and atomic scale

3. Macroscopic investigation is about the classification of crystals by their 3D geometry: pyramidal, cubic, hexagonal prisms with their base pyramids modified by rhombohedral faces = Trigonal, and so on

4. Atomic investigation leads to the classification of the packing of the atoms/ions together. This is strongly influenced by the relative size of the constituent ions and the charge/bonding of each ion/molecule. Crytals can be ionic/atomic/covalent/metallic.

5. Crystal study is essentially a visual topic so it is pointless to use loads of words to describe a crystal structure to explain its behaviour without using explanatory pictures. Examples of packing : http://cst-www.nrl.navy.mil/lattice/. http://www.rockhounds.com/rockshop/xtal/part2.html

6. A particular subtance: atomic /element, compound, can assume different crystalline structures depending on how they form, for example cooling from a molten state. See: http://www.chem.ox.ac.uk/it_lectures/chemistry/mom/pyrite/pyrite.html  Pyrite crystal usually forms as a cube with fine ridges (striations) on the crystal's faces. Less commonly, it forms as octahedrons (eight-sided shapes), nodules, or massive forms

See also: http://www.newton.dep.anl.gov/askasci/chem03/chem03527.htm

[nwkegan]

Thank you for the *delete me*

To rephrase, though you have already partly answered my question, when he dictates that all specimens of the same substance share the same characteristics in their physical manifestation, that's assuming that they take on a crystalline structure, no? That is, they might not always be crystals, but when they are, they will always share that form? The word 'always' is throwing me off a bit, that's all.

[cliverlong]

Is this what you mean?

Sulphur/sulfur can exist in many forms of which two common crystalline forms are orthorhombic or monoclinic see: http://en.wikipedia.org/wiki/Allotropes_of_sulfur

Calcium carbonate can exist as the minerals calcite http://en.wikipedia.org/wiki/Calcite and aragonite  http://en.wikipedia.org/wiki/Aragonite where the crystal structures are different :

Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals
Aragonite's crystal lattice differs from that of calcite, resulting in a different crystal shape, an orthorhombic system

Maleic acid has more than one crystalline form: http://www.rsc.org/chemistryworld/News/2005/December/07120502.asp

See also: http://en.wikipedia.org/wiki/Polymorphism_(materials_science). Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to elemental solids.

Another example is the polymorphs of silica/quartz: http://www.minerals.net/mineral/silicate/tecto/quartz/sio2poly.htm
http://www.science.smith.edu/departments/Geology/Min_jb/SilicaPolymorphs.pdf

Clive

[nwkegan]

Yes! This tells me that his wording was a little too specific, and answers another internal question I had. Thank you for the *delete me* I'm sure I'll be returning sometime in the near future with more questions.