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Polyatomic Ions Flashcards: Names, Formulas and Charges

This set covers the polyatomic ions that general chemistry courses expect you to know, in both directions (name to formula and formula to name), plus the patterns that make them easier to remember. The naming rules follow OpenStax Chemistry, and the ion formulas and charges are the standard textbook values. It is a starter set of the most common ions; your instructor may add a few more, such as borate or silicate.

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Free Polyatomic Ions flashcards: 31-card starter set

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Name to formula (with charge) (15)

Ammonium: formula and charge?

NH4^+. It is the most common polyatomic cation.

Hydronium: formula and charge?

H3O^+, formed when H+ attaches to a water molecule.

Hydroxide: formula and charge?

OH^-

Nitrate: formula and charge?

NO3^-

Sulfate: formula and charge?

SO4^2-

Carbonate: formula and charge?

CO3^2-

Phosphate: formula and charge?

PO4^3-

Acetate: formula and charge?

C2H3O2^- (often written CH3COO^-).

Cyanide: formula and charge?

CN^-

Permanganate: formula and charge?

MnO4^-

Chromate and dichromate: formulas and charges?

Chromate is CrO4^2-; dichromate is Cr2O7^2-.

Oxalate: formula and charge?

C2O4^2-

Thiosulfate: formula and charge?

S2O3^2-

Peroxide: formula and charge?

O2^2-. Do not confuse it with oxide (O^2-).

Mercury(I): formula and charge?

Hg2^2+. The two mercury atoms are bonded together, so it is a polyatomic cation.

Formula to name and the -ate / -ite patterns (10)

What is the difference between -ate and -ite endings?

For the same element the -ate ion has one more oxygen than the -ite ion, with the same charge. Example: nitrate NO3^- and nitrite NO2^-.

Name SO3^2- and give the sulfate formula for comparison.

SO3^2- is sulfite; sulfate is SO4^2-. Both have a 2- charge, but sulfite has one fewer oxygen.

Name the four chlorine oxyanions from ClO^- to ClO4^-.

ClO^- hypochlorite, ClO2^- chlorite, ClO3^- chlorate, ClO4^- perchlorate. All have a 1- charge.

What do the prefixes hypo- and per- indicate in oxyanion names?

Hypo- means fewer oxygens than the -ite ion; per- means more oxygens than the -ate ion. Example: hypochlorite, chlorite, chlorate, perchlorate.

Name HCO3^- and give its other common name.

Hydrogen carbonate, also called bicarbonate. It is carbonate plus one H+, so the charge goes from 2- to 1-.

Name HPO4^2- and H2PO4^-.

HPO4^2- is hydrogen phosphate; H2PO4^- is dihydrogen phosphate. Each added H+ reduces the negative charge by one.

Name HSO4^-.

Hydrogen sulfate (also called bisulfate), 1- charge.

What happens to the charge when you add H+ to a polyatomic anion?

The charge becomes one less negative. PO4^3- becomes HPO4^2-, then H2PO4^-.

Give the charges of carbonate, nitrate, phosphate and sulfate in one line.

Carbonate 2-, nitrate 1-, phosphate 3-, sulfate 2-.

Name SCN^-.

Thiocyanate, 1- charge. It is cyanate (OCN^-) with a sulfur atom in place of the oxygen.

Using polyatomic ions in compounds (6)

When do you put parentheses around a polyatomic ion in a formula?

When you need more than one of the ion in the formula. Example: Ca(NO3)2 has two nitrate ions; NaNO3 needs none.

Write the formula for calcium phosphate.

Ca3(PO4)2. Three Ca^2+ (total +6) balance two PO4^3- (total -6).

Name (NH4)2SO4.

Ammonium sulfate. Two NH4^+ balance one SO4^2-.

Name Al2(SO4)3 and check the charge balance.

Aluminum sulfate. Two Al^3+ (+6) balance three SO4^2- (-6).

Write the formula for sodium hydrogen carbonate (baking soda).

NaHCO3. One Na^+ balances one HCO3^-.

Do you use prefixes such as di- or tri- when naming ionic compounds with polyatomic ions?

No. Name the cation, then the anion. Ca(OH)2 is calcium hydroxide, not calcium dihydroxide.

What this set covers

Covered

About 25 common cations and anions, oxyanion families, hydrogen-containing ions, naming and writing ionic formulas

Best used for

General chemistry nomenclature quizzes and exams

Not covered

Less common ions, transition-metal naming in depth, acid nomenclature rules beyond the basics

Notation

Charges are written with a caret, for example SO4^2- means sulfate with a 2- charge

How to study this set

Polyatomic ions are a vocabulary list with built-in patterns. Learn the anchors first, then the patterns, then use them in compound names.

1
Day 1
Learn the six anchors (ammonium, hydroxide, nitrate, sulfate, carbonate, phosphate). Write each formula and its charge on paper before flipping.
2
Days 2-4
Add the -ite partners and the chlorine family, then the hydrogen-containing ions. Say the pattern out loud ("one fewer oxygen, same charge").
3
Week 2
Do the compound cards: write formulas by balancing charges, then name them back without looking.
4
Final week
Shuffle name-to-formula and formula-to-name cards together. Re-learn any ion you miss twice by writing it ten times with its charge.

Spaced repetition schedules each card to come back just before you would forget it, so reviews get shorter as your recall gets stronger. Cards you miss come back sooner.

Common mistakes to avoid

Learning the formula without the charge

NO3 alone is not an ion. The charge is half of the answer and decides how the compound formula balances.

Confusing the -ate and -ite charges

Nitrite and nitrate have the same 1- charge; the -ite ion just has one less oxygen. Do not change the charge when you change the ending.

Dropping the parentheses

Ca(OH)2 is not CaOH2. Parentheses are needed whenever you have more than one polyatomic ion.

Treating bicarbonate and carbonate as one

HCO3^- and CO3^2- differ by one hydrogen and one unit of charge. Say "hydrogen carbonate" when you are unsure.

Make your own from your notes

A starter set is a head start, not your whole course. Your class notes, slides and textbook are what your teacher actually tests, so turn them into cards too.

Put the ion name on the front and the formula with its charge on the back; make a second card going the other way.
Use your instructor's ion list, not a generic one, so you only drill what the exam covers.
Add compound cards from your homework (for example "name Fe2(SO4)3") rather than only bare ions.
Avoid cards that ask for more than one ion at once, except for obvious pairs like chromate and dichromate.

Generate cards from a topic or notes Turn a PDF into flashcards

Polyatomic Ions flashcards: questions

Do I have to memorize polyatomic ions or can I look them up?

Most general chemistry courses expect memorization for quizzes and exams because every ionic formula and many reaction problems use them. Check whether your instructor provides a reference sheet.

What is the quickest way to remember them?

Learn the anchor ions first (nitrate, sulfate, carbonate, phosphate, hydroxide, ammonium), then derive the others by pattern: -ite means one fewer oxygen and hydrogen versions add one H and one less negative charge.

Why does ammonium have a positive charge when most polyatomic ions are negative?

Ammonium forms when ammonia (NH3) accepts a proton, giving NH4^+. A few polyatomic cations exist (ammonium, hydronium, mercury(I)), but the large majority of the common ones are anions.

What is the difference between sulfate and sulfite?

Sulfate is SO4^2- and sulfite is SO3^2-. Both are 2-; sulfate has one more oxygen.

Does the name of the ion tell me how many oxygens it has?

Only relatively. Within a family, -ate has more oxygens than -ite, but you still need to memorize the -ate formula for each element.

Sources and review

These cards are an independent study aid, not official exam content, and are not affiliated with or endorsed by any exam body or publisher. They were written by hand and checked against the sources below on 2026-10-17. Always confirm details with your course materials and the current official documents.

OpenStax Chemistry 2e: Chemical Nomenclature — Read for the ion tables and naming rules (ammonium, hydroxide, carbonate, phosphate, hydrogen phosphate, sulfate, sulfite, chromate, dichromate, acetate, the -ate/-ite/hypo-/per- pattern, mercury(I), parentheses in formulas). Ions not listed there (hydronium, peroxide, permanganate, oxalate, thiosulfate, cyanide, thiocyanate) are standard textbook values written from general reference.
OpenStax Chemistry 2e: Atomic Structure and Symbolism — Read for the convention that only the first letter of a symbol is capitalized.

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