Lewis dot structure

[blankguy]

Could somebody clarify how to go about drawing a lewis dot structure. So far I got this for each atom the group number from the periodic table is the number of valence electrons(such as IA to VIIIA). Use CO2 and ClO4- as examples. I also know that if there is a negative or positive charge you are suppose to subtract or add electrons from the valence total(- means add + means subtract).

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[flong]

Ill do CO2:

Carbon atom in middle because it is left of oxygen(s). First connect single bonds to both oxygen atoms. The single bonds represent 4 electrons (each bond represent two electrons). Now in the valence shells of the atoms you have 4 electrons(carbon) and 12 electrons(6 for each oxygen). Since you have used 4 electrons to make the bonds you have 12+4-4 or 12 electrons to add to the structure. These electrons sorround the oxygen atoms (6 a piece). Let's now look at the formal charge of the atoms.

Oxygen(s): 6-6-1=-1 <--negative 1 charge on both atoms
Carbon: 4-2-0=+2 charge

So just donate the pair of electrons from each oxygen atom and you have O=C=O with 4 lone electrons around each O atom and a nuetral molecule. It's that simple

 

[blankguy]

flong,
could you elaborate on how you got the double bonds.

 

[dentistinacan]

each atom needs the eight valence to complete it's shell. Therefore, manipulate the electrons to give a complementary eight valence for each atom. Double bonds ( - - ) account for 4.

 

[flong]

yeah, as I said the Oxygen atoms "donate" two electrons each to the carbon atom which forms another bond which yields what is called a "double" bond. So since tge new molecule looks like this:

O=C=O <--we can now look at the formal charges:

Remember the formal charge should be as close to nuetral as possible representing a stable molecule:

So

C: 4-4-0=0 formal chrage
O: 6-2-4=0 formal chrage

On oxygen the 4 represent lone electrons and the two comes from the sharded electrons i.e. two bonds to carbon for each oxygen.

 

[blankguy]

Thanks. I think I got it. Must master this concept 😡

I found a good way to draw the lewis dot diagram.

Make the least electronnegative element the central atom(unless the molecule or ion is diatomic).

Figure out the total number of valence electrons, modify with the charges in the cases of ion(negative charge means adding to the total valence and positive means subtracting).

Start filling out the octet(unless it is H) for the outer atoms then the inner(or central) atom.

If after this the central atom is short of octet, make a double or triple bond to fill up the octet eliminating 1 or 2 unbonded pair from the outer atom from which there is a double or triple bond.

Calculate the charge.

It sounds like a shorthand way without understanding it conceptually but it does seem to work. Anyways I think that understanding will come later (it does for me).

 

[flong]

Here's a question for you, draw the Lewis structure of N2O

 

[blankguy]

Originally posted by flong
Here's a question for you, draw the Lewis structure of N2O

Are you sure its N2O not NO2???

Kinda hard to draw the N2O molecule on this forum.
N has 5 valence electrons for a total of 10 for 2 Ns
and O has 6 valence electrons.

Total valence is 16 electrons.
N2O sounds fishy. But it seems that O the more electronegative element is the central atom since there are 2 Nitrogens.

N-O-N I fill it the outer electrons with the other 6 electrons each since the bond represents 2 electrons. After this is done the central O does not have the octet.

::N=O=N::

The other 2 possibilities(resonance) is having a triple bond one on the left and the other is triple bond on the right and the O with the triple bond having a pair of unbonded electron while the other O has 3 pairs of unbonded electrons.

 

[flong]

nope that is wrong. this one is tricky so think about it.

 

[blankguy]

Originally posted by flong
nope that is wrong. this one is tricky so think about it.

Dam trick questions!😳

::O=N=N::??
O=N-N:::??
-
triple bond

 

[flong]

the first one is right. Don't forget, the least electronegative atom in center

 

[blankguy]

I say something that I don't follow doh!
Flong you made me feel sooooooo stupid😛 😳

Anyways thanks for blindsiding me like that. I'd much rather have this happen here than in the exam.😛

Why not the second one?

 

[flong]

Actually I was blinded by that on my exam 3 years ago! LOL

Man, I can't remember anymore than that from general chemistry.

Um...Try drawing H3PO4

 

[blankguy]

Phosphoric acid, an oxoacid
H3PO4

Central atom is P, 32 valence electrons total
Can't display this properly
H
|
:O:
|
H-::O-P-::O-H
|
:O::

Oxygens are bonded to the P and the Hs to the Oxygens

 

[flong]

WHOA. that is way wrong. omy

look at www.chemfinder.com and search for h3po4 or phosphoric acid

 

[blankguy]

The only thing I got wrong was the double bond.
The spacing for the top and bottom bonds are off, I specified that the H were bonded with Os.