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

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Discussion Overview

The discussion centers around the comparative sizes of a carbon atom and a water molecule, specifically examining how these sizes are measured and the implications of those measurements. Participants explore concepts related to atomic and molecular dimensions, including the van der Waals radius and the methods used to derive size values.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant references a website claiming that the carbon atom is larger than a water molecule based on the van der Waals radius, but questions how the size of the water molecule is determined.
  • Another participant argues that the water molecule cannot be accurately represented by a single radius due to its structure, suggesting that the size quoted is therefore meaningless.
  • A different participant provides information on the van der Waals radius of carbon and water, stating that the values are typically derived from fitting models to experimental data.
  • One participant expresses confusion over the quoted size of 275 pm for the water molecule, mentioning a method involving interpolation of ionic radii from crystal data, but admits to not understanding the explanation.
  • A later post raises questions about the densities of various forms of carbon and compares them to the density of water, suggesting a relationship between size and density.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the size measurements for the carbon atom and water molecule. There is no consensus on the accuracy or meaning of the quoted sizes, and the discussion remains unresolved regarding the implications of these measurements.

Contextual Notes

Limitations include the potential ambiguity in defining the size of a molecule like water, which does not conform to a simple spherical model, and the reliance on various methods for determining atomic and molecular sizes that may not be universally accepted.

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?
 
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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.
 
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.
 
am4th said:
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.
 
Density of diamond is what? Of graphite? Of amorphous carbon? And density of water is how many times less?
 

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