Sulfide, sulfite, sulfate: one ending makes all the difference
Na₂S, Na₂SO₃, and Na₂SO₄ are sodium sulfide, sodium sulfite, and sodium sulfate. The names differ only in their endings, but they are three completely different substances. Lesson 1.5.2 introduced a few polyatomic groups; their English name is polyatomic ions多原子离子, ions made of several atoms that carry a charge as a unit. The AP exam doesn’t give you a table of them, so you need to know the common ones by heart. Fortunately their names follow patterns, and this lesson pairs the patterns with the table to memorize.
Patterns in the names
1. -ate and -ite: when one element forms two oxyanions, the one with more oxygen ends in -ate and the one with one fewer oxygen ends in -ite. Their charges are the same. In Chinese, -ite corresponds to 亚:
- NO₃⁻ nitrate (硝酸根), NO₂⁻ nitrite (亚硝酸根)
- SO₄²⁻ sulfate (硫酸根), SO₃²⁻ sulfite (亚硫酸根)
2. per- and hypo-: chlorine, bromine, and iodine each form four oxyanions. One more oxygen than the -ate ion adds the prefix per- (Chinese 高); one fewer oxygen than the -ite ion adds hypo- (Chinese 次). All four have a charge of 1−:
| Ion | Name | Chinese name |
|---|---|---|
| ClO₄⁻ | perchlorate | 高氯酸根 |
| ClO₃⁻ | chlorate | 氯酸根 |
| ClO₂⁻ | chlorite | 亚氯酸根 |
| ClO⁻ | hypochlorite | 次氯酸根 |
3. hydrogen-: adding an H⁺ to a polyatomic anion adds “hydrogen” to its name and makes the charge one less negative: CO₃²⁻ → HCO₃⁻ hydrogen carbonate (碳酸氢根); PO₄³⁻ → HPO₄²⁻ hydrogen phosphate → H₂PO₄⁻ dihydrogen phosphate. The older name bicarbonate for HCO₃⁻ is still common: baking soda is sodium bicarbonate.
4. Exceptions: most polyatomic ions are anions ending in -ate or -ite, but OH⁻ hydroxide, CN⁻ cyanide, and O₂²⁻ peroxide end in -ide, and NH₄⁺ ammonium is a cation. A few just have to be memorized: MnO₄⁻ permanganate, CrO₄²⁻ chromate, Cr₂O₇²⁻ dichromate, and CH₃COO⁻ acetate.
Here is the table to memorize. Check “Hide names,” say each name as you look at the formula, then click to check; or let the quiz test you:
Chlorine oxyanions
Sulfur
Nitrogen
Carbon
Phosphorus
Containing a metal
Others
Acetate, CH₃COO⁻, is also often written C₂H₃O₂⁻.
Oxyanions of one element: the more oxygen, the "higher" the prefix
Bromine and iodine follow the same pattern: BrO₃⁻ is bromate and IO₄⁻ is periodate. Sulfur and nitrogen have only -ate and -ite: SO₄²⁻ is sulfate and SO₃²⁻ is sulfite. Note that -ate marks the most common member of each family, not a fixed number of oxygens: sulfate has 4 O, but nitrate has only 3.
Compounds containing polyatomic ions
Name these just like binary ionic compounds: cation name, then anion name, keeping each polyatomic ion’s name unchanged. Formulas also balance the charges, with one extra rule: when there is more than one of a polyatomic ion, put it in parentheses and write the subscript outside.
| Formula | Name | Chinese name |
|---|---|---|
| Ca(NO₃)₂ | calcium nitrate | 硝酸钙 |
| (NH₄)₂SO₄ | ammonium sulfate | 硫酸铵 |
| Al₂(SO₄)₃ | aluminum sulfate | 硫酸铝 |
| Fe(OH)₃ | iron(III) hydroxide | 氢氧化铁 |
| NaHCO₃ | sodium hydrogen carbonate | 碳酸氢钠 |
| Na₂HPO₄ | sodium hydrogen phosphate | 磷酸氢二钠 |
In Chinese, salts of oxyanions are named “某酸某” and hydroxides “氢氧化某.” For easily confused pairs such as Na₂HPO₄ and NaH₂PO₄, the Chinese name also states the number of metal atoms: 磷酸氢二钠 and 磷酸二氢钠.
Now practice with compounds that contain polyatomic ions, watching the parentheses:
Example 1: From formulas to names
Name Fe₂(SO₄)₃ and NH₄NO₃.
-
Spot the polyatomic ions. Fe₂(SO₄)₃ contains SO₄²⁻, sulfate. NH₄NO₃ contains NH₄⁺ (ammonium) and NO₃⁻ (nitrate), both polyatomic.
-
Find iron’s charge. The 3 outside the parentheses means three SO₄²⁻. Let iron’s charge be x:
It’s Fe³⁺.
-
Write the names. Fe₂(SO₄)₃ is iron(III) sulfate; NH₄NO₃ is ammonium nitrate.
-
Note. Polyatomic ion names don’t change, and ionic names take no counting prefixes. Iron has more than one possible charge, so it gets a Roman numeral; ammonium and nitrate don’t.
Example 2: From names to formulas
Write the formulas of calcium phosphate and potassium dichromate.
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Calcium phosphate: write the ions. Ca²⁺ and PO₄³⁻.
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Match the numbers. The least common multiple of 2 and 3 is 6: three Ca²⁺ and two PO₄³⁻. With more than one phosphate, add parentheses:
-
Potassium dichromate. K⁺ and Cr₂O₇²⁻, so it takes two K⁺:
There is only one dichromate ion, so no parentheses.
-
A common mistake. Changing the subscripts inside a polyatomic ion, such as writing Ca₃(PO₄)₂ as Ca₃P₂O₈, or reading the 2 in Cr₂O₇ as “two ions.” A polyatomic ion is a unit; its count goes outside the parentheses.
- nitrate
NO₃⁻.
- sulfate
SO₄²⁻.
- phosphate
PO₄³⁻.
- ammonium
NH₄⁺, the only common polyatomic cation.
- carbonate
CO₃²⁻.
- hydrogen carbonate
HCO₃⁻: adding H⁺ to CO₃²⁻ changes the charge from 2− to 1−.
- permanganate
MnO₄⁻.
- dichromate
Cr₂O₇²⁻.
- hypochlorite
ClO⁻; all four chlorine oxyanions are 1−.
Common mistakes
Mistake 1: “Na₂SO₄ is sodium sulfide.” Sulfide is the monatomic S²⁻. Na₂SO₄ contains the polyatomic SO₄²⁻, sulfate, so it is sodium sulfate. An -ide ending usually means a single element (hydroxide, cyanide, and peroxide are the few exceptions).
Mistake 2: “With two polyatomic ions, just add a subscript.” Writing calcium nitrate as CaNO₃₂ is wrong: the 2 would look like it belongs to O. Write Ca(NO₃)₂. Don’t merge it into CaN₂O₆ either, which hides the two nitrate ions.
Mistake 3: “An -ite ion has one less charge than the -ate ion.” The -ite ion has one fewer oxygen atom, but the same charge: SO₄²⁻ and SO₃²⁻ are both 2−, and NO₃⁻ and NO₂⁻ are both 1−.
Quick check
What is the formula of ammonium phosphate?
NH₄⁺ and PO₄³⁻: it takes three ammonium ions to balance one phosphate's 3−, and with more than one ammonium you need parentheses: (NH₄)₃PO₄.
What is the name of KClO₃?
ClO₃⁻ is chlorate. Chloride is Cl⁻, chlorite is ClO₂⁻, and perchlorate is ClO₄⁻.
Summary
- Oxyanions of one element: per-…-ate, -ate, -ite, hypo-…-ite, with oxygen decreasing and the charge unchanged; the Chinese equivalents are 高, (none), 亚, and 次.
- Adding H⁺ adds “hydrogen” to the name and makes the charge one less negative: HCO₃⁻, HSO₄⁻, HPO₄²⁻, H₂PO₄⁻.
- Compounds with polyatomic ions: cation name, then anion name. In formulas, use parentheses when there is more than one of a polyatomic ion, and never change the subscripts inside it.
On to AP (for later)
The AP exam’s reference sheet has no table of polyatomic ions, yet questions use names like sodium hypochlorite, potassium permanganate, and ammonium nitrate directly. Learn this table well enough to write the formula and charge as soon as you see the name; equations, solubility, and acid–base chemistry all depend on it.