Polyatomic ions

Contents

1. Introduction What are polyatomic ions? Why learn about them? What's in this guide?
2. Beginner's guide to polyatomic ions Ions any first year student should know.
1. Arranged by family
2. Common naming guidelines Remembering a few prefixes and suffixes makes learning the lists much simpler.
3. Arranged by charge
4. Quiz on polyatomic ions
3. Resources and references

Introduction

Polyatomic ions are charged molecules. The atoms within a polyatomic ion are usually very tightly bound together, so the ion retains its identity within ionic compounds and over the course of many chemical reactions.

Because polyatomic ions are basic building blocks of so many ionic compounds, learning the names, charges, and formulas of the most common polyatomic ions is absolutely essential before many other skills can be mastered. Those skills include:

• writing down empirical formulas for ionic compounds
• naming ionic compounds
• reading and correctly interpreting labels on reagent bottles
• naming inorganic acids
• predicting the solubility of an ionic compound
• predicting the products of a reaction between aqueous ionic compounds
• predicting the products of neutralization reactions
• writing and balancing ionic equations
• writing and balancing redox equations
• understanding environmental chemistry (e. g. mechanisms in acid rain formation and water quality assessment)
• understanding geochemistry (e. g. composition and formation of minerals)
• understanding clinical and biological chemistry (e. g. electrolyte balance and buffering in blood)

First year students are often asked to memorize lists of polyatomic ions without any context. Mnemonics can be helpful, but they provide an artificial way to organizing what must be memorized. This beginner's guide to polyatomic ions will eventually provide a relevant context for students learning the names and formulas of polyatomic ions by answering these questions: Where do polyatomic ions come from? Where are they found in nature? Why are they important? What do they do? What do they look like? How are they used?

Beginner's guide to polyatomic ions

The following tables list polyatomic ions that any first year student should know. (Common monoatomic ions can be found elsewhere).

Ions arranged by family

Polyatomic cations other than ammonium, hydronium, and mercury(I) aren't usually encountered in general chemistry.

Most common polyatomic anions occur in "families". All members of the family share the same central element and the same charge. There are three common types of variations within the family:

• Different members of the family can have numbers of oxygens.
• Each member of the family can combine with hydrogen ions to partially neutralize their negative charge.
• Some members of the family can have sulfur substituted for oxygen.
Other variations exist but are less common.

Table of common polyatomic cations, arranged by family. Alternate names are given in italics. Select the name of the ion for information about its occurrence, uses, properties, and structure. Blank entries are uncommon or unstable; for a complete table see the Field Guide to Polyatomic Ions.

carbon	nitrogen	sulfur	chlorine
CO32- carbonate HCO3- hydrogen carbonate (bicarbonate)	NO3- nitrate NO2- nitrite	SO42- sulfate SO32- sulfite S2O32- thiosulfate HSO4- hydrogen sulfate (bisulfate) HSO3- hydrogen sulfite (bisulfite)	ClO4- perchlorate ClO3- chlorate ClO2- chlorite ClO- hypochlorite

phosphorus	cyanide	cations	metal oxyanions
PO43- phosphate HPO42- hydrogen phosphate H2PO4- dihydrogen phosphate	CN- cyanide OCN- cyanate SCN- thiocyanate	NH4+ ammonium H3O+ hydronium Hg22+ mercury(I)	CrO42- chromate Cr2O72- dichromate MnO4- permanganate

oxygen	organics
OH- hydroxide O22- peroxide	C2H3O2- acetate

Common naming practices

Polyatomic ions that don't appear on the above tables do NOT always follow these naming practices. If you can remember the formula of the ion whose name ends with ate, you can usually work out the formulas of the other family members as follows:

modify stem name with:	meaning	examples
-ate	a common form, containing oxygen	chlorate, ClO3- nitrate, NO3- sulfate, SO42-
-ite	one less oxygen than -ate form	chlorite, ClO2- sulfite, SO32- nitrite, NO2-
per-, -ate	same charge, but contains one more oxygen than -ate form	perchlorate, ClO4- perbromate, BrO4-
hypo-, -ite	same charge, but contains one less oxygen than the -ite form	hypochlorite, ClO- hypobromite, BrO-
thio-	replace an O with an S	thiosulfate, S2O32- thiosulfite, S2O22-

Some anions can capture hydrogen ions. For example, carbonate (CO₃^2- can capture an H⁺ to produce hydrogen carbonate HCO₃^- (often called bicarbonate). Each captured hydrogen neutralizes one minus charge on the anion.

modify stem name with:	meaning	examples
hydrogen or bi-	(1) captured H+ ions	hydrogen carbonate, HCO3- (a.k.a. bicarbonate) hydrogen sulfate, HSO4- (a.k.a. bisulfate)
dihydrogen	(2) captured H+ ions	dihydrogen phosphate, H2PO4-