How to Determine the Molecular Formula of Oxyanions?

[mmapcpro]

This is going to seem like a very elementary question.

Let's look at a phosphate ion for example.

1) How can I tell by looking at the name that it's an oxyanion? I know it's a polyatomic ion with phosporous.

2) Since it actually IS an oxyanion of phosphorous and oxygen, how do I know how many of each element are in the molecular formula?

My chemistry professor just told me to not worry about it because he's providing a chart of the cations and anions for the exam, but I'm trying to figure out what the concept is here that I'm missing. I want to take the DAT or MCAT next year, and I know we won't have any such charts to take the test with.

Can someone help me out?

 

[GCT]

1) How can I tell by looking at the name that it's an oxyanion? I know it's a polyatomic ion with phosporous.

You just need to be familiar with the names of polyatomic ions.

2) Since it actually IS an oxyanion of phosphorous and oxygen, how do I know how many of each element are in the molecular formula?

There are no standard ways of naming polyatomic ions. You just need to know that it is an oxyanion. Don't worry, you will become familiar with the major polyatomic ions as you progress through chapters. Just become familiar with chemistry and you will become familiar with polyatomic ions.

Taking the MCAT? Names of the polyatomic ions is the least thing you should be worried about. You need to be very familiar with chemistry. You seem very far behind.

 

[mmapcpro]

Thanks for your input.

I don't think you understand my question.

I am looking for the deeper concept behind the naming convention. There has to be a reason why the phosphate ion has 4 oxygen atoms bonded to the phosphorous ion.

I'm completely comfortable with using the common anions and cations, and nomenclature concepts from the book.

However, nobody seems to know the answer to the question. What was the original concept behind the naming convention? The best I've gotten so far is that you just pick up these things by experience, but there has to have been some particular strategy when they were originally named.

Am I making sense?

 

[Bystander]

"-ite," "-ate," "per-(root)-ate" indicate increasing oxidation states of the root atom. Rhyme or reason? At the time such names were hung on these compounds, probably, but the expansion of the field has far outrun their utility for any "systematic" naming conventions.

ClO^- --- hypochlorite, -1
ClO₃^- --- chlorate, -5
ClO₄^- --- perchlorate, -7
SO₃^-2 --- sulfite, -4
SO₄^-2 --- sulfate, -6;

no rhyme, no reason, just a ranking of oxidation numbers.

 

[mmapcpro]

SO4 = sulfate (standard)
SO3 = sulfite (one less)
NO3 = nitrate (standard)
NO2 = nitrite (one less)
PO4 = phosphate (standard)

of course we can compare oxidation states between similar ions...but there's no pattern across different ions.

so, based on your theory, SO4, NO3, and PO4 were probably the first ions of each of these polyatomic combination to be seen, and thus were given the -ate suffix...then subsequent ions were named appropriately in comparison to that based on increasing or decreasing oxidation state...makes sense...

so there's essentially no way to know these without having either seen them in a chart or used them previously.

like I said, I'm familiar with most of the common anions and cations and how the naming works...I was just trying to dig deeper beyond what I was being told to see if there was some kind of underlying theme...but it seems as though that theme has become diluted over time as more and more ions and compounds become known.

thanks