Covalent structures and bonding
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Please explain about Van der Waals radii and covalent radii. I don’t understand them at all.
Ulex writes…
We have to start by realising how difficult it is to answer an apparently simple question: ‘How big is an atom?’ It is difficult to answer this question because although it is a fair assumption to take the atom as a sphere, it isn’t a hard object like a snooker ball. Not only is it soft but the electrons which make up most of it are in motion and are likely to be found at all sorts of distances from the nucleus. It is therefore virtually impossible to tell where exactly the outer surface of an atom is or even if it has an outer surface. Even if you could tell, how could you measure its position practically?
Although it isn’t possible to measure the size of a single atom, it is possible to measure the distance apart of two nuclei. If you measure the distance apart of two chlorine nuclei in a chlorine molecule and halve this distance, you have an indication of how big the chlorine atom is. This measurement is called the covalent radius of a chlorine atom.
There is another way of estimating atomic size which is to measure how far apart two nuclei of the same element are when they are not bonded to each other but simply bump into each other. Their electronic clouds repel each other so that they don’t get as close as they do if they are combined by a covalent bond. Half of this distance of closest approach is called the van der Waals radius of the atom.
The difference between the two estimates of radius is illustrated in a diagram at the top of page 212 in Nuffield Students’ Book (4th edition).
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updated: 02 July 2003
