Carbon atom larger than a water molecule? Measuring a molecule's size?

  • Thread starter am4th
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So there's a website here:

https://learn.genetics.utah.edu/content/cells/scale/
That shows the scale of various entities (coffee bean, viruses, a carbon atom, a water molecule etc). It says that the carbon atom is larger than the water molecule. This is based on the 'van der waal radius'; however, it doesn't say how the water molecule's size (radius?) is measured.

From my understanding of what I've just read, the 'van der waal radius' is the distance between two atoms (carbon in this case) when they are at their most stable (so the potential energy between them is at its lowest?). Is that correct?

They are quoting a size of 275pm for the water molecule. Does anyone know how this value is reached?
 

Answers and Replies

Borek
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There is definitely something wrong. You can approximate atom as a sphere and sphere is indeed characterized by a single number (radius). That's not the case with water molecule, so the number given is meaningless.
 
TeethWhitener
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The vdW radius is a non-covalent measure. In graphite, for example, the distance between adjacent graphene sheets is about 335 pm, so dividing this by two (~170 pm) would be a decent approximate value for vdW radius of carbon. The vdW radius of water is usually quoted at around 140-150pm. These values are usually ascertained by fitting a calculated water model to experimentally observed parameters.
 
OmCheeto
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They are quoting a size of 275pm for the water molecule. Does anyone know how this value is reached?
I found that number at one website, listed as 2.75 angstroms, but I don't understand the explanation; "The atomic diameter can be determined from interpolation of the effective ionic radii of the isoelectronic ions (from crystal data) of O2- (2.80 Å), OH- (2.74 Å) and H3O+ (2.76 Å) "

Good luck.

ps. Just bought my first microscope, and am very grateful for the cell size and scale site link.
pps. Just did a bunch of maths, and I don't get it either.
2019.11.17.pf.atoms.sizes.in.the.real.world.png

In my mind, real world water molecules, are bigger.
 
Bystander
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Density of diamond is what? Of graphite? Of amorphous carbon? And density of water is how many times less?
 

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