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Polyatomic Ions

Written by tutor Laurie C.

Polyatomic ions are just what their names imply. They are many (“poly”) atomed (“atomic”) ions. Simply, they are ions with more than one element in them. If you’re reading this then you still may not be seeing difference between a regular ion and a polyatomic ion. So let’s look at a regular ion and a poly atomic ion side-by-side.

Regular Ion: Chlorine Ion Polyatomic Ion: Chlorate
Cl- ClO3-

The regular ion has just one element, Chlorine, in it. The polyatomic ion has two elements, Chlorine and Oxygen, in it. A good way to recognize that there are two elements in the polyatomic is to see that there are two capital letters in the ion’s formula. All the elements on the periodic table start with a capital letter and only some of them have a second letter that is lower case. So if you see two capital letters together in a ion then you will know that it is a polyatomic. Just like capitals are used to shout in texts, polyatomic ions shout at you to recognize they are there.

Your chemistry teacher or tutor have been bugging the quarks out of you to memorize or at least know the polyatomic ions by sight so you can look them up on a table provided in your exams. Let’s see a list of common polyatomic ions and how they can be grouped to make them easier to recognize. I like to group them by charge and then by patterns in naming that we will discuss next.

Word Formula      Chemical Formula
Charge +1    
Ammonium NH4+
Charge -1    
Acetate C2H3O2- (Sometimes looks like CH3COOH-)
Perchlorate ClO4-
Chlorate ClO3-
Chlorite ClO2-
Hypochlorite ClO-
Nitrate NO3-
Nitrite NO2-
Cyanide CN-
Hydroxide OH-
Permanganate MnO4-
Charge -2    
Carbonate CO32-
Chromate CrO42-
Dichromate Cr2O72-
Sulfate SO42-
Sulfite SO32-
Charge -3    
Phosphate PO43-

These little pains in the beaker are one of the most frustrating things to deal with in beginning chemistry due to the similar sounding but different –ite and –ate endings. Students try to attribute a number of oxygen atoms to these –ite and –ate endings but that is not the case. Just check out the difference between Nitrate, Nitrite, Sulfate and Sulfite. Let’s think of them as two different families of polyatomic ions.

Nitrogen family                      Sulfur family                
Nitrite Nitrate      Sulfite      Sulfate
NO2- NO3-      SO32-      SO42-

After examining these two families of polyatomic ions we can come up with a general rule for the –ite and –ate endings.

  • The –ite ending is given to the polyatomic ion family member with fewer oxygen atoms (think of them being the crazy family member who has a little less wits than the others)
  • The –ate ending is given to the polyatomic ion family member with more oxygen atoms (the rocket scientist of the family who has the most wits)

Most of the polyatomics are easy to memorize except for the Chlorine, Nitrogen, and Sulfur families. Those are the ones that have the interesting family dynamics. Everyone else in the polyatomic bunch are pretty average. Let’s examine the Chlorine family.

Chlorine Family                        
Perchlorate Chlorate      Chlorite      Hypochlorite
ClO4- ClO3- ClO2- ClO-

We can look at the Chlorine family as all siblings made by Father Chlorine and Mother Oxygen. There are four children in the Chlorine family, they all have a negative charge (You can think of the negative sign as making them all blond haired or some physical attribute that allows you to pick them out of a crowd with ease). You can remember them easier as people that you give personalities too. Let’s say that Perchlorate is the eldest and is currently attending MIT to get a degree in rocket science so he can lead the first expedition to mine asteroids. Make up any story you like so that you can remember them because, just like people you meet at parties, you will get them mixed up as you are meeting them now all together but will run into them later separately.

Other than those three families the rest of the polyatomic ions have to be recognized by memorization alone. Just treat them all like people you have met at a party. On flashcards you can even draw up little pictures of what each of these guests look like so as to help you remember them. One thing is for sure, your success in chemistry will rest heavily on being able to recognize them again when they confront you on exams.

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