# Memorizing polyatomic ions? Using Periodic Table

In my Chemistry course, we must memorize a list of common polyatomic ions. Is their an easy way of memorizing ions such as Sulfate $\ce{SO4^2-}$ by looking at just the periodic table. I listed the ones we have to memorize bellow. I know that if they contain oxygen (which is most of them) they usually end in "ate" or "ite." But how do I know how many Oxygen an ion will contain and its charge. My teacher said the only reference table we can use on our test is the Periodic Table of Elements.

I have to memorize the name and the formulas of the ions. Any methods will be much appreciated. I guess the real question is their any trends in the periodic table that helps predict the charge and number of oxygens and polyatomic ions will have? So if I have Sulfate for example, Can I predict the formula by looking at Sulfur on the periodic table

Update: My question is not a duplicate of When to use -ate and -ite for naming oxyanions? because my question wants to **use the periodic table to identify and memorize polyatomic ion **. Voldemorts question had nothing to do with memorization what so ever!

But, what I don't understand is how does the book know that NO−3 is Nitrate: how does the book know that NO−3 is "the most common oxyanion for the element". How does it know that a charge of −1 and 3 oxygen atoms create "the most common" Nitrogen oxyanion?

His question had nothing do with specifically looking for patterns in the periodic table to help him or her memorize the polyatomic ions. In fact he or she wasn't even asking for a technique or method to memorize polyatomic ions, but rather figure out which form of a polyatomic ion is "most common." For Example: He wanted to find out how the author of his book knows $\ce{NO3^-}$ is more common than $\ce{NO2^-}$

• Grouping it by charge is a horribly inefficient way of looking at things. You should look at the individual groups in the Periodic Table and observe the common oxidation states. Then you will notice that they tend to be 2 apart: for example, the halogens have -1 ($\ce{Cl-}$), +1 ($\ce{ClO-}$), +3 ($\ce{ClO2-}$), +5 ($\ce{ClO3-}$), +7 ($\ce{ClO4-}$). This is also true for other elements. Also, there are some exotic species there. I don't understand why you should be forced to memorise them... – orthocresol Oct 18 '15 at 23:36
• @orthocresol I didn't group it by charge. The table above was the table that my professor gave me of poly-atomic ions we are expected to know. But, I will look into individual groups in the periodic table and look for patterns in the oxidation state. – James Smith Oct 18 '15 at 23:39
• Not that exotic, also hypophosphite is (H2PO2)- 2 hydrogens don't dissotiate. – Mithoron Oct 18 '15 at 23:41
• @Mithoron His question doesn't ask for help using the periodic table to memorize a list of polyatomic ions. My question is on how I can use the periodic table to memorize a list of poyatmoic ions. – James Smith Oct 19 '15 at 1:10
• Nowhere did I say that you were the one who grouped by charge. I just said it's inefficient. Your teacher gave you a list of ions, including the nonexistent PO3(3-) and PO2(2-), grouped by charge to study. – orthocresol Oct 19 '15 at 7:14

## 2 Answers

Number of oxygens:

If your anion is in:

• the 2nd period;
• the VIIth main group/the 17th group/a halogen; or
• is silicon:

then the -ate anion will have three oxygens.

All other -ate elemental anions will have four oxygens.

(Take note of ‘aluminate’ which I haven’t physically seen in that form yet but your teacher seems to insist be $\ce{AlO2-}$; disregarding this general trend.)

From the -ate anion, remove one oxygen to arrive at the -ite, remove two for hypo-ite. Add one for per-ate.

Charge:

Start at an anion with the most oxygens. (Note: Usually this is the -ate but because the halogens and permanganate are special they have the per-ate anion which is more oxygen-rich and important here.) Assume the maximum possible oxidation state for the non-oxygen atom. Assume $-\mathrm{II}$ oxidation state for every oxygen. Add up and take the negative value to arrive at the charge.

A hydrogen- will add $\ce{H}$ and lower the charge by one.

A thio- means one oxygen is replaced by sulphur.

A di- means take two of the (hydrogenated) anion and subtract water. Pyro means the same thing.

Memorise all the remaining! That is most importantly:

• Chromite. Similar to aluminate I’ not sure if I ever saw it out in the real world.
• Cyanide/cyanate. Thiocyanate is just like thiosulphate.
• Oxalate, acetate and tartrate. Organic ions that will never fit into this scheme nicely.
• Peroxide and superoxide. But those are -ides anyway.
• Permanganate; which is almost like a perhalogenate. Luckily, molybdate and chromate fall into a category described above.

Periodic Table

This is about the charges of polyatomic ions, determined by the number of valence electrons (i.e. group number in the periodic table) brought by all the atoms in the ion:

If the polyatomic ion contains an odd number of atoms from elements with odd atomic number the charge of the polyatomic ion is odd.

Otherwise the charge is even. The most common charges for anions are -1 and -2, and for cations +1 and +2.

https://www.papazyan.org/oddoddodd_rule_for_polyatomic.html

• Are there any exceptions? – Karsten Theis May 15 '19 at 16:23
• @KarstenTheis Thanks for copying the rule over here. The only exceptions should be (unstable; reactive) radical ions with an odd number of electrons. – Dr. P. May 15 '19 at 16:35