[Richthofen]

According to the periodic table I recieved in class, potassium has an ionic charge of -1, while the polyatomic ion chromate has a charge of -2.
If you balanced them out and wrote an equation, why is it not K.₂CrO.₄, but K₂Cr.₂O.₇?
Not, the numbers are subscripts.

[savoy7]

Potassium is in group I - it has a +1 oxidation number.

You've presented 2 polyatomic ions: Dichromate, Cr2O7-2, and chromate, CrO4-2. Chromium has 3 different oxidation states: 6,3 & 2.  In both dichromate and chromate, Chromium has the oxidation number of +6.  
Oxygen has an oxidation number of -2.  In both polyatomic ions, the net charge is -2.


Potassium dichromate (di stands for 2 chromium) is different than potassium chromate (1 chromium), so you get K2Cr2O7 and KCrO4.

I hope this answers your question...unless your question is:  Under what conditions does Cr go into the dichromate form and under what conditions does it go into the chromate form.

savoy

[Richthofen]

Potassium is in group I - it has a +1 oxidation number.

You've presented 2 polyatomic ions: Dichromate, Cr2O7-2, and chromate, CrO4-2. Chromium has 3 different oxidation states: 6,3 & 2.  In both dichromate and chromate, Chromium has the oxidation number of +6.  
Oxygen has an oxidation number of -2.  In both polyatomic ions, the net charge is -2.


Potassium dichromate (di stands for 2 chromium) is different than potassium chromate (1 chromium), so you get K2Cr2O7 and KCrO4.

I hope this answers your question...unless your question is:  Under what conditions does Cr go into the dichromate form and under what conditions does it go into the chromate form.

savoy

Sorry, I misinterpreted myself. The first formula I typed out should be Potassium Dichromate, but after checking what you typed out, it still does not make sense. I would have thought that there would be an even number of oxygen instead of an odd number.

From what I learned in science, dichromate would be (CrO4)2 and not K2Cr2O7. So how does that work out?

[AWK]

There are many elements that form polyacids,ie: P, S,  CR, W, and many others.
In the case of Cr - two acids are well known (but others also exists, at least in the form of salts), namely H2CrO4 and H2Cr2O7. The former is present in neutral and basic medium, the later in acidic one.
2H2CrO4 = H2Cr2O7 + H2O (this is an equlibrium reaction)

Oxidation number for Cr in both acids is +6.

[savoy7]

And now I know.  Thanks