Question About Lewis Dot Structures

[Rigaudon 0]

Hi everybody,

I'm taking Pre-IB chemistry this year and our teacher just went over Lewis Dot Structures. They're pretty easy, but I have a question about it. As some of you may have noticed, I'm not the kind of person who can stand doing something and not understand what or why I'm doing it. I haven't gotten a chance to ask my teacher yet since she doesn't answer many questions and walks out right after class. Anyways:

I'll use a simple example: Phosphate (PO4 with a -3 charge) We were told to add up all valence electrons and then add the electrons of the charge. Phosphate has 5, oxygen has 6 (times 4 is 24), and the charge is 3, for a total of 32. The Lewis Dot Structure is easy enough to draw.

My Question: I see the Lewis Dot Structure as how the valence electrons between atoms interact and form covalent bonds. All the electrons added together are obviously more than 32, since valence electrons are only the outer level. However, if it only has 29 valence electrons readily available from the valence electrons of the atoms (Phosphate is 5 + Oxygen (6)*4), where do the extra 3 come from? I know I'm supposed to add the 3 from the charge, but the charge doesn't add more valence electrons to the atoms, as far as I know. Are they drawn from a lower energy level? Do they appear magically just to make me confused? I WANT ANSWERS!

Thanks in advance.

Edited April 15, 2010 by Rigaudon (see edit history)

[linekill 1]

You are actually referring to a Polyatomic Ion in an aquaeous solution, PO4 3-, phosphate ion. The question is valid. The extra 3 electrons that you have to add to complete the Lewis Electron Dot structure aren't drawn from the lower energy levels, they're actually from the Hydrogen atoms in the solution. When you put phosphoric acid, H3PO4, in an water it undergoes several stages of dissociation reactions leading to the creation of the Phosphate Ion, PO4 3-.The reaction looks like this: H3PO4 <----> H+ + H2PO−4H2PO−4 <----> H+ + HPO2−4 HPO2−4 <-----> H+ + PO3−By the way, I used the symbol "<------>" to signify, "In equilibrium with".

[Rigaudon 0]

The question is valid. The extra 3 electrons that you have to add to complete the Lewis Electron Dot structure aren't drawn from the lower energy levels, they're actually from the Hydrogen atoms in the solution. When you put phosphoric acid, H3PO4, in an water it undergoes several stages of dissociation reactions leading to the creation of the Phosphate Ion, PO4 3-.
The reaction looks like this:

H3PO4 <----> H+ + H2PO−4
H2PO−4 <----> H+ + HPO2−4
HPO2−4 <-----> H+ + PO3−

Ohh... I see. Well, I mean that makes more sense than magically appearing electrons. If this is the case, I assume it's like this for every polyatomic ion then?

When you say "in the solution", does that mean that polyatomic ions cannot exist outside of a given environment, or solution? Or is this the only way to get said ion?

[linekill 1]

Ohh... I see. Well, I mean that makes more sense than magically appearing electrons. If this is the case, I assume it's like this for every polyatomic ion then?

Yes, that's right. But the path on how the dissociation reaction occurs may differ.

When you say "in the solution", does that mean that polyatomic ions cannot exist outside of a given environment, or solution? Or is this the only way to get said ion?

I referred to the example as being in a solution (an aqueous solution, to be specific) so it would be easier to understand. There are complicated solutions in nature that could not be described as an aqueous solution but still a solution.

Another way to look at it aside from "in the solution" point of view is taking into consideration that they are in an Ionic Bond (A bond formed by an attraction between two different charges, positive and negative.) For your case, PO4 3- and 2 H +.