[sam12103]

While I work on practice problems, in particular Lewis diagrams, always when I check my answer either I placed a bonding pair when it wasn’t necessary or even sometimes I put lone pairs but in the answer it shows that the lone pairs have been substituted to bonding pairs.

I am not going to show you all the problems that I make this mistake with, but it happens soo frequently. It gets especially annoying when I am trying to find the formal charge because my answer turns out to be incorrect, or when I have to find the electron pair geometry or the molecular geometry, when I am given only a compound. This is soo annoying and confusing.

Can you please give me a tip to master Lewis diagrams and not to make this mistake anymore.

Thank you

[renge ishyo]

For Lewis structures, first make a table listing the number of valence electrons you have. Here's an example for NCl₃.

Valence e's
---------------
N - 5 e's
3(Cl) - 3(7e's) = 21 e's

Total = 26 e's

Next, place the atom with the lowest number of atoms in the molecular formula in the center (generally speaking). In this case this is nitrogen which has only one atom in the formula as opposed to chlorine which has three atoms in the formula. Place the other 3 chlorine atoms surrounding the nitrogen.

You have a total of 26e's to add to the structure. First add bonds from each nitrogen to each chlorine. Each bond has 2 electrons in it so you have a total of 20 e's left. Next, go to each of the outer atoms (NOT the central atom until last!!!) and place electrons around that atom until its octet is satisfied. For each of the three chlorine atoms, place six electrons on the outside as lone pairs. Once you are done with this you will see you have 2 e's left. Take any left over e's and place them on the central atom. So the two e's go on Nitrogen. You are done. The next step is to find the electron-pair and molecular geometry as described in the other post  

[sam12103]

I made a mistake on another problem, I followed the procedure but when I checked, I got another incorrect Lewis Diagram

It says I have to draw the Lewis Diagram for CO2
I drew it as you said: fill up the outer atoms first with the lone pairs, and then if there are any remainders, take them to the central atom.
When I did it, I came up with this
   . .             . .
 : O   -- C --  O : 
    . .          . .

The answer in the back of the book was


   . .             . .
   O     = C =  O   
   . .           . .

Can you please explain what mistake I made?

Thank you very much for your help. Unfortunately my professor doesn’t explain this concept well.

[renge ishyo]

In your first picture, did you satisfy the octet for the central carbon atom? It wants 8 electrons, but it only has 4 in the first picture you drew. If you take some of the lone pairs on the oxygen and make two more bonds to carbon then you can have the octet satisfied on each atom. That is your goal.