Jordan Hammond
I’m a software developer by trade, not a chemist. However: I’m also a lifelong learner who knows quite a bit about memorizing things, and that includes memorizing polyatomic ions. So that’s what I’m here to help you do.
And because I don’t have formal chemistry training, I can explain what I do know in layman’s terms.
I promise: Once you’re done reading this, you’ll have a better foundation that’ll make it easier to memorize polyatomic ions. You’ll also have access to a nifty new tool that’ll help you quiz yourself.
Create Free Polyatomic Ion Quizzes
What Really Makes Polyatomic Ions so Hard to Memorize?
This is important, because understanding why they’re challenging is how you get to the bottom of solving the problem.
Now, we all know polyatomic ions are crucial in understanding chemical reactions and properties, and mastering them is essential for any student of chemistry. You might also know that each polyatomic ion consists of two or more atoms covalently bonded together. Unlike monatomic ions, which consist of a single atom, polyatomic ions include several atoms, usually with oxygen as a common element.
It’s probably easier if I just show you:
Take the polyatomic ion, Carbonate and it’s symbol (CO₃²⁻). It’s made up of one carbon atom (C) and three oxygen atoms (O). Hence the CO₃. Easy, right?
But why the -2 in CO₃²⁻?
Carbon, as an atom, usually has a charge of +4, because that’s how many more protons it has than electrons. Oxygen typically has a charge of -2 for the same reason (it has 2 fewer protons than electrons). In the case of Carbonate, you’re looking at one carbon (+4) and three oxygen atoms (-2 x 3= -6). What’s 4 +(-6)? Answer: -2. That’s why you get the little 2- at the end of the equation, and end up with CO₃²⁻.
Simple enough so far, yes? The problem is this:
- You may not have memorized the charge of every single atom on the periodic table.
- Carbonate, as a word, doesn’t necessarily scream “Hey! I have one carbon atom and three oxygen atoms!”)
- The charge can change based on the oxidation state (a bit more on this later), and various other exceptions.
- There are so, so many polyatomic ions.
Which brings us to how to solve this problem …
Making It Easier to Remember Polyatomic Ions
Really what you’re dealing with here is layers of memory. You have to remember:
- The symbol for each atom.
- Which atoms correspond to which words (e.g., carbon and oxygen in the case of carbonate).
- How many instances of each atom there are (one carbon, three oxygen).
- The charge of those atoms.
It sounds like a lot, and it is, and that’s why so many people struggle. The trick to making this easier on yourself is to find little memory hooks, little mental placeholders that minimize the amount of actual memorization you need to do.
Here’s how that works:
1. Learning the atomic symbols
These are some of the most commonly used atoms in polyatomic ions: P, N, O, I, Cl, Br, C, S, H, Mn, Cr.
Remembering the symbols is actually super easy. With the exception of Mn and Cl, all of these represent either the first letter or first two letters of the atom (C is Carbon, Br is Bromine, N is Nitrogen, H is Hydrogen, and so on).
You can most easily memorize these 11 elements in 10 minutes or less.
2. Find hints that will help you match the polyatomic names with their symbols
Nitrate, for instance, sounds an awful lot like nitrogen. Sulfite sounds like sulfur. Those are pretty straightforward clues of what the first symbol is (N and S, respectively).
“Ite” and “ate” are also big tells: They always indicate oxygen is present. Already, you know that nitrate has an NO and sulfate has an SO. Now you just need the number of atoms, and the charge.
It isn’t always this simple. Acetate (C₂H₃O₂⁻ or CH₃COO⁻) and ammonium (NH₄+) are not great clues. But again, the goal here is to minimize the amount of memorization you have to do.
3. Learning how many instances of each atom there are.
This is trickier, and requires more rote memorization. But here are some tips I find extremely helpful:
- “Ate” always has one more oxygen atom than “ite” for ions in the same family. So, sulfate (SO₄²⁻) has one more oxygen than sulfite (SO₃²⁻). Similarly, nitrate (NO3-) has one more oxygen than nitrite (NO2-). Memorize just the “ates, and you are also effectively memorizing the ites, and vice versa.
- “Di” means two units (CO2 is carbon dioxide because of the two oxygen atoms).
- “Bi” just means there is a hydrogen ion tacked on, as in bicarbonate (HCO₃⁻), which is the same thing as hydrogen carbonate.
You’ll inevitably encounter “per” and “hypo” at some point. “Per” indicates the highest oxidation state (meaning the highest charge). “Hypo” indicates the lowest oxidation state (meaning the lowest charge).
Huh?
I know, This part is confusing. See, the thing about charges is that they are not always the same, even for the same atom …
4. Memorizing charges
While most atoms have a typical charge, charges can change based on oxidation state. Per and hypo can be clues, but they’re not clear tells.
And again, this is where minimizing the amount of things to remember really helps. Take sulfate, for instance. We know it has sulfur (because duh) and oxygen (because “ate”). We also know “ate” always has more oxygen than “ite.” We’re already starting with SO. Now all you really need to remember is the number of oxygen atoms and the charge. You get: SO₄²⁻. If
And because you know that sulfite has less oxygen than sulfate, you know the subscript next to O has to be 3 or less, and it is: (SO₃²⁻).
Now, you may be wondering: Why is the charge the same if there is one fewer oxygen atom? There’s a logical explanation for this elsewhere on the internet, surely. All you need to know for the purposes of memorization is to be distrustful of charges. You have to memorize these little hellions the old-fashioned way.
Crucial Tips for Memorization
Just by reading this, and re-reading it, you’ll start memorizing things faster with the base I’ve given you.
But you also have to do the rote memorization and spaced repetition to commit it all to memory. For that part, I have two tactics I highly recommend:
1. Multi-colored Flashcards: Each color can represent a grouping of polyatomic ions (bromine polyatomic ions are blue, sulfur is yellow, etc.). Put the name of the ion on one side and its chemical formula and charge on the other. This method leverages active recall, requiring you to remember the information each time you see the name of the ion. Review these flashcards over and over again.
2. Practice Quizzes: This is SO important. Flashcards are great, but they’re controlled. You need to actually create quizzes that will truly randomize questions. For this, I highly recommend generating a study set using DorothyMemoryApp. It’s free, easy and it’s smart. I recommend making these as far in advance of your test as possible. They actually know when you’ll forget something based on how close you were to the correct answer, and they re-quiz you based on your performance.
Confidence Is Key
Memorizing polyatomic ions is a fundamental skill in chemistry that aids in understanding more complex chemical concepts and reactions.
They’re also a royal pain in the tuchus.
But by minimizing the amount of memorization you actually have to do, and by then employing techniques such as flashcards and quizzes, you really can enhance your memory and confidence in dealing with these ions.
Remember, every chemistry student faces challenges with memorization. If they did it, so can you. It just takes practice, smart work and the right tools.