Is there any physical difference between a single crystal and a molecule? I was just reading an article and it mentioned single crystal silicon for making wafers and it made me think of a program I watched once about plastic which noted that in the forming process, the heat can cause the constituents to combine into one big molecule. Is there any difference between inter and intra molecular bonds?
Thanks for your help,
Rob
When people say a “single crystal”, what they really mean is a single grain. In other words, all the molecules are in the same lattice. If you can immagine a bunch of marbles in an aquarium, and they are spaced regularly (with all the gaps being the same), that would constitute a “single crystal”, if you will. Each individual marble would be a molecule. The thermoset plastics you speak of are most likely amorphous, that is, they don’t exibit a lattice structure (like glass).
Plastic polymers can be considered a single molecule (Wiki agrees if that makes a difference - Polymer - Wikipedia ) They aren’t crystals, so I’ll shush.
Are you saying that molecules in a crystal aren’t bound to one another?
Thanks,
Rob
A crystal doesn’t have to contain molecules.
For example, a crystal of ClNa doesn’t contain molecules of ClNa. It contains as much Cl- and Na+, distributed in such a way that each ion is surrounded by 6 of the other one, but no “Cl-Na” molecule per se; each Cl- sees each of its surrounding Na+ as absolutely identical and viceversa. There isn’t “a Na+ that’s more my Na+ than the rest”.
You can have molecules in any physical state. Solid, liquid, solution, suspension, gas or plasma.
A crystal is always a solid.
A molecule is a structure with a beginnind and end, no repetibility* and a certain degree of flexibility (more or less flexibility depending on the physical state). [*There can be repeated fragments, for example any protein repeats the 20 basic aminoacids many times along with other optional stuff… but the repeatibility never reaches the level of a crystal, not even in polymers formed by a single “building block”.]
A crystal contains a “basic structure” called a crystal cell which repeats itself over and over in 3D.
The immense majority of the properties of the crystal don’t change with its size - the properties of, for example, a string of identical aminoacids, are very different when it’s 10 aAs long or when it’s 1000. Adding a handful of aAs at the end of an actual protein can change its behaviour completely.
If a crystal contains molecules, the atoms in molecule A will be bound more strongly to the rest of the aroms in molecule A than to the atoms in molecule B. When you take an Xray picture of a sugar crystal and process it (the original picture looks like a very badly-done piece of Op Art), you get the positions of the atoms in the crystal’s cell, and you actually see sugar molecules, with distances between atoms very similar to the distances they have when the sugar is dissolved in water. (I haven’t done X-ray interpretation myself but I do have some background in turning the numbers into the pics and in comparing numbers obtained in different situations, measuring the flexibility, etc)
A crystal is what you get when atoms or molecules arrange themselves in a regular manner. Regular meaning that all of them are oriented in the same direction and spaced equally far apart. Whether or not the crystal is one single molecule will depend on what the components are. If the crystal is composed of silicon atoms, then yes, it is one large molecule. Same for carbon in its diamond form. If it’s something like sodium chloride, then the crystal is made up of equal numbers of sodium and chlorine ions. If the crystal is of some organic substance (e.g., “crystal” meth), then the crystal is composed of individual methedrine molecules.
The same type of molecule (or atom) can exist in different crystal forms. This has to do with how the individual molecules orient with respect to one another. Graphite and diamond are good examples of this. Both are pure carbon, but have completely different appearances and properties.
I’m pulling this from very distant memory, so if I make a mistake someone correct me.
In general,
covalent bonds (molecular bonds as one would most easily understand them) are shorter and stronger than
ionic bonds (like you have in ClNa crystals), and these are stronger than
hydrogen bonds, and these are stronger than
instantaneous bonds (born when the electrons in adjacent molecules dance around, creating charges that last a very short time).
The bonds you get in complexes (a kind of compound which is not a molecule, where a central metallic atom is surrounded by fragments or molecules that donate electrons to said metallic atom) can be stronger or weaker than ionic bonds. In a ClNa solution in water, the ions are complexed with water molecules but it’s a relatively weak complex (it forms and breaks apart easily); OTOH the complexes of the Fe in hemogoblin to CO or CN- are extremely strong (it’s the mechanism by which CO and cyanide kill, their complexes with hemogoblin are stronger than those of CO2 and O2 so you asphyxiate).
Graphite and diamond don’t contain molecules, although the layers of graphite and the totality of the atoms in diamond are bound covalently. There isn’t such a thing as a graphite molecule outside a crystal.
The point I was trying to make is that crystals can either by made up of molecules or atoms/ions. This doesn’t mean that a crystal is one big molecule. You can’t have a molecule of diamond or graphite (either by itself or in solution). But if the crystal is a molecular one, it is usually relatively easy to “uncrystalize” them.
Another detail is that crystals don’t have to be solid. There is quite a large industry based on “liquid crystals”.