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by amoeba 6058 days ago
Best I can tell, they have it wrong. They might be interpreting size differently than I expect but Carbon should be around 220pm and Water around 310pm.

Rough estimates for diameter are based on molecular mass and density; density being variable. I'm curious how they did their calculations.
1 comments
araneae 6058 days ago
I'm not sure if it's actually wrong, but rather a function of how they draw atoms versus molecules.

Diameter is a funny concept for atoms and molecules because electrons orbit at different distances around the nucleus probabilistically. The depiction of this carbon atom I think represents this, with the lighter color areas denoting areas of lower probability.

However, drawing such a diagram for a molecule isn't very pretty, because the orbitals for each atom in the molecule overlap. They probably drew the water molecule using intermolecular distances, and then filled in the distances so that each atom was a sphere. In actuality the electrons are found with some probability outside this area.

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amoeba 6058 days ago
If they're drawing Carbon with respect to its orbitals then they are completely wrong.

Carbon has both S and P orbitals whereas water has hybridized orbitals. This would mean both their carbon and water are drawn incorrectly.

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araneae 6057 days ago
Actually, as I look at carbon more closely, that's exactly what they're drawing. The inner circle (s orbital) has two yellow dots (electrons) on it, and the outer circle (p orbital) has 4 yellow dots. It does look like they totally ripped off this lady's drawing though: http://www.dorlingkindersley-uk.co.uk/nf/ClipArt/Image/0,,_1...

Clearly they aren't drawing the orbitals for water, which I already stated.

But you're right, carbon is probably too big. Wolfram alpha says the diameter of carbon is 134pm, and we all know WA is right about everything.

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araneae 6057 days ago
So, I actually submitted feedback about this on the site. Their response:

"Thanks for your response to our website.

Depicting the size of atoms definitely has its challenges. We've been getting a lot of comments and feedback about this.

The radius of a carbon atom is not a measure of the size of its electron cloud, but rather the distance between the nuclei of two adjacent atoms. See http://www.chemguide.co.uk/atoms/properties/atradius.html The size of the carbon atom in our activity is based on its van der Waals radius (170 pm), not the covalent or ionic radius (around 77 pm). See http://www.webelements.com/carbon/atom_sizes.html

The sizes of the atoms in the water molecule have been adjusted to make them consistent with other atoms depicted in the activity, although the overall diameter of the molecule is the same.

Sincerely,

Sheila Avery Genetic Science Learning Center"

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