# Polyatomic ions

by scott_alexsk
Tags: ions, polyatomic
Sci Advisor
P: 985
 Quote by scott_alexsk I thought that the first explaination sounded werid anyways. So how does the carbonate get the -2 charge? Is it that there are monoatomic ions making up carbonate, if so which, or does it gain electrons from a different source? -Scott
If the ions were monoatomic, they wouldn't be polyatomic. Carbonate is polyatomic
 P: 354 "I would conclude that monoatomic ions are not involved, but that's just my opinion."-ShawnD Now that I am certain that carbonate has 2 more electrons than protons, hence the negative two charge, in what way does it obtain these electrons? Is it that it forms bonds with two monoatomic oxygen ions with a negative 1 charge, each, or is it something else as ShawnD suggests. How would carbonate gain electrons in your opinion? It is clear to me that it cannot simply just magically take electrons from another atom and remain unbonded, if Gukul and the others are incorrect, how do you justify your prior response ShawnD? I should have made myself more clear my prior post. -Scott
Sci Advisor
P: 985
 Quote by scott_alexsk It is clear to me that it cannot simply just magically take electrons from another atom and remain unbonded
That's exactly what happens.
 P: 354 So I am incorrect? -Scott
 Sci Advisor P: 985 I'm not really sure if you're correct (still a bit confused here). A friend suggested to me that you might be thinking of oxidation states. In that case, carbon is +4 and each oxygen is -2. Carbonate has 1 carbon and 3 oxygens so it's +4 -2(3) = official charge of -2. Does that help?
 P: 354 The reason I started this thread in the first place, was because when I asked my chemistry teacher how polyatomic ions formed with a charge, but without being bonded to some other atom, she gave me a strange answer. I mean what element does carbonate get the 2- from? It cannot just steal the electrons from nothing, there has to be some other element it gets these electrons from. Also where does carbonate occur in nature? My chemistry teacher gave me a werid answer that it does not gain any extra electrons, but it becomes a prevailing negative 2 charge because of the bonds. Basically it seems that everyone here has defeated that arguement (repeatedly), so how does carbonate occur? What chemical reactions form it and how common is it? Thanks, -Scott
 P: 274 Carbonates are very common in nature. Calcium carbonate is the main component of limestone, which makes up about 10% of all sedimentary rock. Carbonates make up a whole class of minerals as well, see http://en.wikipedia.org/wiki/Carbonate_minerals When carbon dioxide from the atmosphere dissolves in water, a dilute solution of carbonic acid is formed. That's why natural (not acid rain) rainwater is slightly acidic (pH of around 6). The carbonic acid then slowly leaches rocks, forming carbonates.
Sci Advisor
P: 985
 Quote by scott_alexsk so how does carbonate occur? What chemical reactions form it and how common is it?
I answered that one the first page, but I'll make it more readable.

Start with CO2
Add water to get H2CO3
A hydrogen leaves to get H+ and HCO3-
Another hydrogen leaves to get 2H+ and CO3--

It's a very common reaction. It's why pop tastes better when it's not flat.
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P: 985
 Quote by bomba923 $$\begin{gathered} {\text{CO}}_2 \left( g \right) + {\text{H}}_2 {\text{O}}\left( \ell \right) \rightleftharpoons {\text{H}}_2 {\text{CO}}_3 \hfill \\ {\text{H}}_2 {\text{CO}}_3 + {\text{H}}_2 {\text{O}} \rightleftharpoons {\text{H}}_3 {\text{O}}^ + + {{\text{HCO}}_3} ^ - \hfill \\ {{\text{HCO}}_3} ^ - + {\text{H}}_2 {\text{O}} \rightleftharpoons {\text{H}}_3 {\text{O}}^ + + {{\text{CO}}_3} ^{2 - } \hfill \\ \end{gathered}$$
This series of equilibrium reactions is one of the buffer systems in every mamal on the planet.
 P: 354 Thanks guys, I finally was able to get that cleared up. I appreciate your time. -Scott
P: 354
 You must see that the atoms can't be both charged and neutral at the same time. The carbonate ion is charged due to it having 2 extra electrons - no amount of double bond moving will remove this charge.
OK. I am resurrecting this thread. I have found that my own book directly contradicts LordOfBaals 'conservation charge'. It lists the first example of a polyatomic ion as being hydroxide.

 The oxygen atom is bonded covalently to the hydrogean atom. The hydrogean atom is stable with two electrons in its outer level. The hydrogean atom contributes only one electron to the octet of oxygen. The other electron required for oxygen to have a stable octet is the one that gives the -1 charge to the ion.
Unless this has changed since 1998, you guys are wrong. I would appreciate someone responding who actually knows what he or she is talking about. To reitterate, my question is "How can there be a charge
if there is a equal number of protons and electrons, in a polyatomic ion?"
-Scott
 P: 3 i think all this polyatomic this and mono atomic that has prevented a lot of readers from understanding what you are looking for. Hydroxide is indeed a 'polyatomic' ion - I usually call it an ion but hey. It is comprised of a hydrogen atom and an oxygen atom that has 1 extra electron - that gives it the 1- charge does this picture help you get it? Attached Thumbnails
Sci Advisor
P: 882
 Quote by scott_alexsk I would appreciate someone responding who actually knows what he or she is talking about. To reitterate, my question is "How can there be a charge if there is a equal number of protons and electrons, in a polyatomic ion?" -Scott
To think that the people who have responded dont know what they are talking about is just wrong, some a quite knowledgeable (and it is best not to push people away that are trying to help).

As LordOfBaal said,
 You must see that the atoms can't be both charged and neutral at the same time. The carbonate ion is charged due to it having 2 extra electrons - no amount of double bond moving will remove this charge.
This is inline with your book, saying that the Hydroxide ion (and more specifically, the Oxygen side of the Hydroxide) picked up an extra electron.
When the extra electron is picked up (or lost), the number of protons and electrons are not equal, and there is an imbalance in the charge. In Hydroxide's case, there is a -1 charge.
 P: 354 Sorry about that. You guys explained it. Its just that when I have looked at the diagrams that model polyatomic ions before, I couldn't tell the number of electrons in the model, because for extra electrons would be marked with a 'X'. I didn't know this counted as an electron. Just to let you know, your explainations always made sense, but I thought that there was more information I was not getting. Thanks and sorry, -Scott

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