Polyatomic Ions: Charges Explained by Chem Class

[scott_alexsk]

Currently in my Chemsitry class we are reviewing these ions. Since these ions are formed of perfectly neutral elements, how can they have a charge as a whole. My teacher told me these charges in the anions can be attributed to an incomplete oxygen bond such as the negative 2 charge in carbonate( CO subscript 3, superscript 2), but why is this?
-Scott

 

[Gokul43201]

These are no different from mono-atomic ions - one or more of the atoms in the radical is not neutral.

So...polyatomic ions are not made up of neutral atoms. And rarely is there a definciency of electrons (incompleteness) in a bond within such an ion. if at all, such a deficiency will give rise to a net positive charge (taking away electrons makes a thing positive), and hence will not explain the 2- charge on the carbonate ion.

 

[scott_alexsk]

Then why does the carbonate polyatomic ion have a net charge of negative two? Are you saying that naturally the carbon exsists with a -2 charge. If so how does the carbon every find itself with this and remain unbonded such as in a monotomic ion? (By the way in what frequency do ions occur in nature?) Also carbon forms a negative four charge in carbide (supersript -4). If this is true then why does not this carry over to carbonate? How can carbon have an effinity for 4 electrons then suddenly only have 2 extra? Why does not carbonate according to this reasoning not exsist with a negative 4 charge?
-Scott

 

[Gokul43201]

This follows directly from drawing the electron dot-structure (or Lewis structure) for the CO₃^2- ion. Have you learned how to do these ?

 

[scott_alexsk]

Yes of course. Proceed with your explanation of why this still has a neagtive two charge.
-Scott

 

[Gokul43201]

It (the extra 2- charge) comes from the O-atoms wanting to have octet structures, without denying the central C-atom its own octet. O being highly electronegative (having a high propensity to accept electrons in order to achieve an octet) will easily take on a negative charge to maintain this octet.

     .. (-)
    : O :
 ..   |   ..
: O - C = O :
 ..
 (-)

 

[scott_alexsk]

That makes sense since the electrons are essentially caught inbetween the two atoms, mostly out of range of the protons producing a net negative charge. But using the same principle why is not the double bond oxygen forms with carbon represented with 2 additional negative charges with the third oxygen? Or does the carbon in the bonding have a +2 charge? What is a carbon ion in that form called anyways?
-Scott

 

[Gokul43201]

Carbon almost never has a 2+ charge. In rare cases, you have carbocations with a 1+ charge.

Your questions show that you need to first learn how a lewis structure is constructed and the basis for this construction. Without this understanding, this discussion can go nowhere.

 

[scott_alexsk]

I think I have a fair enough understanding, its just I do not understand which atom in the sole double bond has the electrons. Is it the carbon or the oxygen that has the electrons which are being shared. In the diagram I see two shared electrons between carbon, with two other electrons, total four, and oxygen, with 4 other electrons, total six. One way or the other a certain element is giving electrons in this bond, either as an error or something I do not understand. In order for there to be any reason in the sharing of the electrons either the carbon or oxygen atom have to have two less electrons than they do naturally in nature. That is why I mentioned that incredibly strange carbon atom with a two postive charge. Either that or the oxygen has to have a 2+ charge for there to be a bond. I appreciate your time in helping me understand this.
-Scott