Chemistry - Lewis Structure question

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Discussion Overview

The discussion revolves around the Lewis structure of the molecule H2CO, focusing on the placement of atoms and the implications of different structural representations. Participants explore the relationship between Lewis structures and molecular geometry, including considerations of bond angles and repulsion between atoms.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant expresses confusion about where to place atoms in the Lewis structure of H2CO and questions the validity of their own drawn scenarios compared to the book's answer.
  • Some participants argue that all three proposed Lewis structures are equivalent, although they acknowledge potential issues with molecular shape and bond angles.
  • Another participant suggests that the left and center drawings are preferable as they illustrate the repulsion between hydrogen atoms, which are both positively charged.
  • A counterpoint is raised regarding the spatial arrangement of hydrogen atoms, suggesting that they are not as far apart as initially thought due to the influence of the negatively charged oxygen atom.
  • Participants discuss the relevance of VSEPR theory in understanding molecular geometry, noting that Lewis structures do not account for it.

Areas of Agreement / Disagreement

There is general agreement among participants that the three Lewis structures are equivalent, but there is disagreement regarding the implications of atomic placement and the influence of molecular geometry on bond angles.

Contextual Notes

Participants note that the Lewis structures do not fully represent the molecular geometry as predicted by VSEPR theory, and there are unresolved questions about the actual bond angles in the molecule.

Who May Find This Useful

This discussion may be useful for students learning about Lewis structures, molecular geometry, and the relationship between atomic charge and spatial arrangement in chemical compounds.

mystmyst
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I understand the idea of the Lewis Structure and the octet rule. What I don't know is where to place the atoms. And unfortunately my book doesn't explain it. They just draw the lewis structur and expect us to understand how they did it.

For example: Draw the lewis structure of H_2CO

[PLAIN]http://img46.imageshack.us/img46/7392/lewisstructure.png

The book's answer is in black. Two other scenarios I drew in red and blue. Are my other scenarios incorrect? Does where you place the outer atoms really matter?

Can someone please help?
Thanks!
 
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I would say all three are equivalent.

Edit: there is a slight problem with the molecule shape and bond angles, but if someone wants to be nitpicky about it, all three are wrong (including the one from the book). IMHO it is better to draw both hydrogen bonds on one line, to show that they are equivalent. In reality all three bond angles are close to 120 deg.
 
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all the three are equivalent indeed!
 
All three are equivalent. Lewis structures are simple drawings, they don't take into account VSEPR theory.
 
They are all correct, but the left drawing and the center drawings are the best out of all three. They show that the hydrogen's are spread as far apart from each other as possible because both of those atoms are positively charged, so they repel each other.
 
Sniperman724 said:
hydrogen's are spread as far apart from each other as possible because both of those atoms are positively charged, so they repel each other.

No, they are not spread apart as far as possible, they are very close to the sp2 angle of 120°.

Note that your logic is incomplete. If they are positively charged, oxygen must be charged negatively - and its charge must be twice higher, as the molecule is neutral. That in turn means repulsion from oxygen is stronger than repulsion between hydrogens and in effect they are not spread apart, but a little bit closer than expected.

Not sure what the real angle is, but quick googling shows this page:

http://www.800mainstreet.com/formaldehyde/Formaldehyde-Bond-length.html
 

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