Chemistry - Lewis Structure question

In summary: The bond lengths in formaldehyde are shorter than those in water because of the electron withdrawing ability of oxygen.
  • #1
mystmyst
57
0
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 [tex]H_2CO[/tex]

[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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  • #2
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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  • #3
all the three are equivalent indeed!
 
  • #4
All three are equivalent. Lewis structures are simple drawings, they don't take into account VSEPR theory.
 
  • #5
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.
 
  • #6
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
 

1. What is a Lewis structure?

A Lewis structure is a visual representation of the valence electrons and bonds in a molecule. It shows the individual atoms, their bonding arrangement, and the overall charge of the molecule.

2. How do you draw a Lewis structure?

To draw a Lewis structure, you first need to determine the total number of valence electrons in the molecule. Then, arrange the atoms so that each atom is surrounded by its valence electrons. Connect the atoms with single bonds, and distribute any remaining electrons as lone pairs to satisfy the octet rule.

3. What is the octet rule?

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with 8 valence electrons. This is often seen in molecules where all atoms have a full outer shell, such as in noble gases.

4. What is the purpose of a Lewis structure?

The purpose of a Lewis structure is to show the bonding and electron arrangement in a molecule. It can also help predict the shape, polarity, and reactivity of a molecule.

5. Are there exceptions to the octet rule?

Yes, there are exceptions to the octet rule. Elements in the third period or higher can have an expanded octet, meaning they can have more than 8 valence electrons. This is seen in molecules such as PCl5 or SF6. There are also molecules with an odd number of valence electrons, such as NO or NO2, which do not follow the octet rule.

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