MadSci Network: Chemistry
Query:

Re: crystal formation

Area: Chemistry
Posted By: Maria Gelabert, staff, Rutgers University
Date: Sat May 10 12:20:25 1997
Area of science: Chemistry
ID: 858183211.Ch
Message:
The basic force of attraction that makes atoms/ions to come together into
an ordered solid is electronic or coulombic.  There are many, many issues
involved with crystal growth, so I will try to address some of the basics.

For an ionic solid, like NaCl (table salt),
in the crystal structure are Na+ and Cl- arranged in an ordered fashion.  
The lattice energy is large, since these + and - charges are coulombically attracted
towards one another.  These species also exist in solution just before crystal 
growth.  When do crystals grow?  For every solid, there a 
solubility; this is the maximum amount of solid you can dissolve in water.  
Salt is very soluble in water, but there is however a maximum.  When this point 
is reached, solid begins to come out of solution in the form of crystals.  
How big the crystals are depends on the rate at which this happens.

Another way to grow crystals is by melting a chemical and resolidifying it by
slow-cooling.  Now we must distinguish between molecular and extended solids.
Let's take sulfur, a molecular solid.  Its crystal structure consists of S8
crowns held together by weak forces called van der Waals bonding.  If you heat
it to 150 C (it melts at 120), the weak forces break down and we have liquid
consisting of long chains of sulfur atoms.
Slow-cooling through the melting point would allow enough time for the 
molecules to reassemble in an ordered fashion, whereas a fast-cool would yield 
a glassy, noncrystalline blob.  For an extended solid, let's talk about salt
again, NaCl.  When it melts (I do not know the temp, but it's high), you 
have a sea of Na+ and Cl-.  Upon cooling, they come together again mostly
because of coulombic attraction.  In a different extended solid, like gold 
metal, the gold molecules come together because of electron orbital overlap.  
For both NaCl and Au, once again the rate of cooling determines the size
and degree of order in the crystals. 




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