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

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SUMMARY

The discussion centers on the differing molecular shapes of elements in column 14 of the periodic table, specifically comparing carbon's CH3 radical to other elements like silicon, germanium, tin, and lead. While carbon exhibits a planar triangular shape due to the distribution of its unpaired electron, the other elements maintain a pyramidal shape. The VSEPR theory primarily addresses electron pair repulsion, which does not adequately explain the behavior of unpaired electrons in these radicals. Various theories, including steric repulsion and Bent's rule, have been proposed to explain these differences, but none provide a comprehensive solution.

PREREQUISITES
  • Understanding of VSEPR theory and its application to molecular geometry.
  • Familiarity with the concept of unpaired electrons in radical species.
  • Knowledge of hybridization and its influence on molecular shape.
  • Basic grasp of steric effects in molecular interactions.
NEXT STEPS
  • Research the implications of VSEPR theory on molecular shapes beyond electron pairs.
  • Explore the role of unpaired electrons in determining molecular geometry.
  • Investigate Bent's rule and its applications in molecular chemistry.
  • Examine the steric effects in various alkyl radicals and their influence on shape.
USEFUL FOR

Chemistry students, molecular modelers, and researchers interested in molecular geometry and radical chemistry will benefit from this discussion.

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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
 
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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).
 

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