# Lewis Structures

1. Sep 28, 2004

### decibel

i'm having trouble drawing up some lewis structures related to VSPER theory.
i was given 22 Lewis structures to draw and i had problems with these ones...

1 - [N(CHsubscript3)subscript4]^+
2 - IClsubscript2
3 - [IClsubscript4]^-

thanks if anyone can help out with these questions.

2. Sep 28, 2004

### Pyrrhus

Try visiting the Chemistry forum in Other Sciences, i remember doing this exercices, but a long time ago

3. Sep 28, 2004

### Sirus

Were you taught the 5-step method for drawing Lewis structures?

4. Sep 29, 2004

### maverick280857

Clarke's Method

Hi

Here's something known as the Clarke method for drawing Lewis Structures. I read it from a General Chemistry book by Dr. Hubbard (I forget the complete name). I've found it very effective and thought I should share it with you folks.

1. Find the TOTAL number of electrons ($$n_{e}$$) for a species. For $$ICl_{4}^{-}$$ this would be 7+4(7)+1 = 36.
2. Next compute the quantity (6y+2) where y is the total number of atoms minus hydrogen (for $$H_{2}O$$ this would be 1).
3. Compare $$n_{e}$$ and (6y+2):

(a) if $$n_{e}$$ < (6y+2) then the species contains a double bond. For ICl4-, 6y+2 = 32 which is less than 36 by 4 so there is either a triple bond or two double bonds (and you can see that there are actually 2 double bonds here).

(b) if $$n_{e}$$ = 6y+2, the structure contains only single bonds. As an example consider the water molecule. For it, $$n_{e} = 8$$ and 6y+2 = 8.

(c) if $$n_{e}$$ > (6y+2) the central atom has an expanded octet (which means it cannot belong to the second period as the second period elements do not have d-orbitals). For example, $$IF_{7}$$ has $$n_{e} = 7+7(7) = 56$$ and 6y+2 = 6(8)+2 = 50.

Note that this method will only take you a bit further in making the structure but won't tell you the crucial information: that of connectivities. You must then use formal charges and some experience to rule out incorrect structures if more than one is possible.

Cheers
Vivek