[elad]

Hi,

Why can't I draw this lewis structure for no3-?
                             : O  :
                            ..    ||     ..
                         : O   - N  =O
                            ..            ..
(in this lewis structure the formal charge on 2 O atoms is 0 and the formal chrge on the third O is -1, and the formal charge on the N atom is 0.

I found this lewis structure in youtube:
                                ..
                             : O  :
                            ..    |     ..
                         : O   - N  =O
                            ..            ..
But I didn't understand why my lewis structure is incorrect.

Thanks,
Elad

[mjc123]

Because it has five bonds to nitrogen

[mikasaur]

As mjc123 states, your initial guess is incorrect because nitrogen cannot have 5 bonds.

Nitrogen is a period-2 element and so always follows the octet rule. It will never have more than 4 pairs of valence electrons.

Period-3 elements like P and S and Cl are sometimes observed breaking the octet rule, e.g. PCl₅. This is because of the phenomenon of an expanded octet. Basically, the energy level of the 3d orbitals is low enough that they can participate in bonding. You can start learning more about it here.

You might wonder why nitrate doesn't have the following form:

This is because the nitrate ion is a classic example of resonance. There is a lone pair of electrons that kind of "float around" the molecule (they're delocalized), creating bonds that are not quite single and not quite double bonds between the N and all the O.

Because of all of this, we get the structure for [itex]\ce{NO3-}[/itex]

[mikasaur]

Oh and I should say the more accurate structure is this one:

Here the N has +1 charge because each O is "donating" -1/3 of a charge to it. There are no single or double bonds but something in between.