# Chemistry - Lewis Structure question

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 [Broken]

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?

Thanks!

Last edited by a moderator:

Related Biology and Chemistry Homework Help News on Phys.org
Borek
Mentor
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.

Last edited:
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.

Borek
Mentor
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