Periodic Table Polyatomic Ions- Reference Chart

What Are Polyatomic Ions?

Polyatomic ions are molecules that carry an electrical charge. Unlike simple ions like Na+ or Cl-, these clusters contain multiple atoms bound together, but they behave as a single unit in chemical reactions.

You need to memorize these. There's no workaround. Chemistry won't wait for you to look them up every single time you write a formula.

Here's the hard truth: if you don't know your common polyatomic ions, you'll fail every equation. Period.

The Complete Polyatomic Ions Reference Chart

Bookmark this. Print it. Memorize the ones marked with asterisks until you can recite them in your sleep.

Positive Polyatomic Ions (Cations)

Name	Formula	Charge
Ammonium*	NH₄⁺	+1
Hydronium	H₃O⁺	+1
Mercury(I)	Hg₂²⁺	+2

Negative Polyatomic Ions (Anions)

Name	Formula	Charge
Hydroxide*	OH^-	-1
Nitrate*	NO₃^-	-1
Nitrite	NO₂^-	-1
Acetate*	CH₃COO^-	-1
Bicarbonate*	HCO₃^-	-1
Permanganate	MnO₄^-	-1
Cyanide	CN^-	-1
Thiocyanate	SCN^-	-1
Hypochlorite	ClO^-	-1
Chlorite	ClO₂^-	-1
Chlorate*	ClO₃^-	-1
Perchlorate	ClO₄^-	-1
Carbonate*	CO₃^2-	-2
Sulfate*	SO₄^2-	-2
Sulfite	SO₃^2-	-2
Chromate	CrO₄^2-	-2
Dichromate	Cr₂O₇^2-	-2
Hydrogen phosphate	HPO₄^2-	-2
Oxalate	C₂O₄^2-	-2
Thiosulfate	S₂O₃^2-	-2
Phosphate*	PO₄^3-	-3
Arsenate	AsO₄^3-	-3
Dihydrogen phosphate	H₂PO₄^-	-1

* = Must-know ions for any chemistry course

The -ate/-ite Pattern

Most polyatomic ions follow a predictable pattern. Learn this once and you'll unlock half the table automatically.

-ate = the most common form of that element (sulfate SO₄, nitrate NO₃)
-ite = one less oxygen than the -ate form (sulfite SO₃, nitrite NO₂)
hypo-___-ite = two less oxygen atoms (hypochlorite ClO)
per-___-ate = one more oxygen atom than -ate (perchlorate ClO₄)

Chlorine Series Example

Prefix	Name	Formula	Oxygen Count
hypo-	Hypochlorite	ClO^-	1
(none)	Chlorite	ClO₂^-	2
(none)	Chlorate	ClO₃^-	3
per-	Perchlorate	ClO₄^-	4

How to Name Compounds with Polyatomic Ions

Rules are simple. Stop overcomplicating this.

Name the cation first — this is usually the metal or positive ion
Name the anion second — drop the ending and add -ide if it's a single atom; keep the polyatomic name if it's a polyatomic ion

Examples

Na₂SO₄ = Sodium sulfate (sodium + sulfate)

Ca(NO₃)₂ = Calcium nitrate (calcium + nitrate)

NH₄Cl = Ammonium chloride (ammonium + chloride)

Fe(OH)₃ = Iron(III) hydroxide (iron + hydroxide)

Notice that polyatomic ions don't change. You don't rename them. You don't drop endings. You just write them as-is.

Getting Started: How to Actually Memorize These

Most students fail at memorization because they try to memorize everything at once. Don't.

Week 1: The Big Four

Start with these ions. They appear in almost every chemistry problem:

NH₄⁺ (ammonium)
OH^- (hydroxide)
NO₃^- (nitrate)
SO₄^2- (sulfate)

Week 2: Add These

CO₃^2- (carbonate)
PO₄^3- (phosphate)
ClO₃^- (chlorate)
CH₃COO^- (acetate)
HCO₃^- (bicarbonate)

Week 3: Fill in the Rest

By now, you'll see the patterns. The -ate/-ite system becomes obvious. You won't need to brute-force memorize the rest.

Common Mistakes to Avoid

Don't drop charges. Polyatomic ions have fixed charges. Na⁺ pairs with OH^-, not OH^2-.
Don't rename polyatomic ions. Sulfate is sulfate. It's not "sulfuride" or whatever you think makes sense.
Don't forget parentheses. Ca(NO₃)₂ has two nitrate ions. Write it out or you'll lose marks.
Don't mix up bicarbonate and carbonate. HCO₃^- has a -1 charge. CO₃^2- has a -2 charge. This matters.

Quick Reference: Charges by Group

If you're stuck, this shortcut helps with common ions:

Ion Type	Common Charges
Most -ate ions	-1 or -2
Hydroxide, bicarbonate, cyanide	-1
Carbonate, sulfate, chromate	-2
Phosphate, arsenate	-3
Ammonium (the only common positive polyatomic)	+1

Bottom Line

You need this reference chart. Print it. Use it. Eventually you'll know these without looking.

There's no secret technique. Flashcards work. Writing formulas works. Repetition works. Whatever method you choose, the only thing that doesn't work is hoping you'll figure it out as you go.