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I have a chemistry test coming up and I might need to know the charge that goes with the different ions like $\ce{SO4}$ has $-2$, $\ce{NO2}$ is $-1$ and $\ce{PO4}$ is $-3$. Is there an easy way to remember this by looking at the periodic table or something like remembering that the second row elements are all going to be $\ce{something-O3}$ and the next row is all $\ce{something-O4}$ for the -ate endings (except $\ce{Cl}$ which is the odd man out). It will make writing out the different acid combos easier, if I can remember the charge difference that needs offsetting by the hydrogen.

I'm just trying to lay down some associative triggers for myself.

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I don't believe there is any simple way to determine what they are except by just plain memorization. I have used some tricks that have helped me.

When I write down my poly-atomic ions, I generally put them in similar groups in a certain order. From least amount of oxygen to most amount oxygen.

  • hypo (name of ion) ite -> least oxygen
  • (name of ion) ite -> one more oxygen than above
  • (name of ion) ate -> one more oxygen than 'ite'
  • per (name of ion) ate -> one more oxygen than 'ate'

Understanding the above helps most with 'ite' and 'ate'. 'Ate' generally being 1 more than 'ite' for oxygen.

Then I list them from least charge to most charge. I order them this way:

Nitr ite $\ce{NO2-}$

Nitr ate $\ce{NO3-}$

Sulf ite $\ce{SO3^2-}$

Sulf ate $\ce{SO4^2-}$

You can see that 'ate' has one more oxygen, and that I have listed them in charge order. Sometimes looking at the trends in oxygen and charge will help you memorize.

I have had to remember them by writing them down several times and now I am comfortable.

Here is another example

Hypochlorite is $\ce{ClO^-}$

So, chlorite should be?

$\ce{ClO2-}$, because it has one more oxygen, and

Chlorate should be?

$\ce{ClO3-}$, because it has one more oxygen than the above, and

$\ce{ClO4-}$, will be called?

Per chlor ate, because it has the maximum amount of oxygen.

The above is not a hard and fast rule. One exception I can think of is manganate and permanganate.

Manganate is $\ce{MnO4^2-}$

Permanganate is $\ce{MnO4^-}$

Same amount of oxygen, just a different charge...

I hope this was helpful.

Also, trying to figure any of the above out from a periodic table didn't make a difference to me because my professor tests us without it. Yikes!

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