Why do some elements in column 14 have different shapes than carbon?

[bjon-07]

The Odd Shapes of Life ( carbon)

Hi, I have a question

CH3, doublet (one unpaired electron) has a triangular planor shape.
According to vspr theory it should have a tiangular pryimidal shape. The rest of the elements in row 14 ( Si, Ge, Sn, Pb) all have a tringular pryimidal shape.

From what i understand, in carbon which is shaped like a "Y" the unpaired electron is spread out equal above and below the the 'plane' which makes up the "2D" three orbitals that take the shape of the Y.

Why don't the other elements in column 14 have this same feature. i have feeling that is has something to do with electron negativey.

thank you

 

[TeethWhitener]

This is actually a very good question, and one that has garnered a lot of attention from researchers. The quick answer would be to say that VSEPR only applies to electron PAIR repulsion, and not unpaired electrons. But the fact is that, while the CH₃⋅ radical is planar, other alkyl radicals are pyramidal (most notably CF₃⋅). There are a lot of different theories on why this is the case, and none of them are totally satisfactory. This paper:
https://pubs.acs.org/doi/10.1021/om950560k
claims that the planarization of the methyl radical is due to steric repulsion (which doesn't make much sense, seeing as how the trifluoromethyl radical and the t-butyl radical are both pyramidal). Other theories claim that Bent's rule drives the pyramidalization of CF₃⋅ (this electrostatic argument is a bit difficult to rationalize, given that the methyl cation is planar--and for straightforward orbital hybridization reasons).