Posted on November 30, 2010 by sciencegeist
And why this would be obvious if you could see individual water molecules
November 29, 2010
One of my favorite observations in chemistry is how symmetry on a molecular scale translates into something we can observe. (That is: really, really, really, really itty-bitty shapes that we can only “see” with super-fancy microscopes become the shapes of materials that we see every day) Take sodium chloride (common table salt, NaCl) for example.
Individual sodium ions and individual chloride ions come together to make tiny little cubes of NaCl.
A rendering of the cubic crystal of sodium (green) chloride (purple). (Image credit)
When a sodium chloride crystal continues to grow and becomes a salt granule that we can hold in our hand, it retains its cubic shape.
Cubic crystals of sodium chloride. (Image credit)
I am continually amazed with this observation. Small shapes keep stacking up to become bigger versions of themselves. I don’t know why this is so striking to me. (Why shouldn’t Nature work this way?) It just is. Structure and symmetry can be incredibly captivating! (For a competing view on the beauty of unstructured materials, check out Joerg Heber’s post today on metallic glasses.)
Fortunately for me, it is getting to be that time of year when I’m about to be surrounded by molecular and macro-structure. The weather is turning colder, there’s a perceptible nip in the air, and there is certainly plenty of snow on its way this winter season.
An individual snowflake (snow crystal). (Image credit)
But why do crystals of water look this way? Well, as is the case with table salt, we can learn a lot by looking at water on the molecular level.
As water crystals (snowflakes) form inside of clouds, the water molecules stack up in a hexagonal pattern.
They stack up this way because the oxygen atoms (red balls) like to share their hydrogen atoms (gray bars) with each other (a phenomenon called hydrogen bonding). The stabilizing forces behind hydrogen bonding orient the water molecules into a hexagonal pattern.(Image credit)
And so we are left with beautiful snowflakes that look like this:
They start breaking off from their general hexagonal shape (while retaining their hexagonal symmetry) because the water molecules in the corners have the least amount of stabilizing hydrogen bonds (only a few neighboring water molecules). And they want more.
So, the corner molecules start pulling in more water molecules. (Image credit)
And we’re graced with beautiful snowflakes that look like this:
For more on snow crystal science, visit snowcrystals.com, which is run by Ken Libbrecht, another fellow Caltech scientist. (FYI, his snowflake books make fantastic Christmas or Hanukkah gifts, especially for the science geek in your life!)
And, just to prove that there are other materials that can invoke the season check out this image and many like it coming from the research of Song Jin at Wisconsin.
Lead Sulfide pine trees.(Image credit: Journal of the American Chemical Society)
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