Is there an easy way to remember charges on ions?

Question

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.

Answer 1

I don't believe there is any simple way to determine what they are except by just plain memorization. I have given some tricks below 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 $\ce{MnO4^2-}$ but permanganate is $\ce{MnO4^-}$; same amount of oxygen, just a different charge.

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

Answer 2

There is a very useful trick that I was taught to remember the charges and formulas:

Nick the Camel ate an Icky Clam Booger for Supper in Phoenix.

This trick helps with most of the polyatomics. The first letter of each word stands for the first letter of there compound.

• Nick Nitrate. The number of consonants is the number of oxygen so. Nick = 3 O's. The number of vowels stand for the charge. One vowel in Nick = -1 charge. So, $\ce{NO3-}$
• Camel Carbonate; 3 consonants= 3 O's; 2 vowels = 2-; $\ce{CO3^2-}$
• Icky Iodate; 3 consonants = 3 O's; 1 vowel = 1-; $\ce{IO3^-}$
• Clam Chlorate; $\ce{ClO3^-}$
• Booger Borate; $\ce{BO3^3-}$
• Supper Sulfate; $\ce{SO4^2-}$
• Phoenix Phosphate; $\ce{PO4^3-}$

Hopefully this helped, you can always add word to the sentence to help remember other polyatomic ions.