C. Chemistry: Chemical Reactions · Chemical ReactionsCompounds with Polyatomic Ions
C3.8 C3.1 C2.1 A1.12About 70 minutes
Learning goals- I can explain what a polyatomic ion is and recognize common ones.
- I can use the -ate/-ite and hydrogen patterns to work out related ions.
- I can write formulas for compounds with polyatomic ions, using brackets correctly.
- I can name compounds with polyatomic ions, including those with multivalent metals, and count their atoms.
Words to know
polyatomic ion
A group of covalently bonded atoms that has an overall charge, e.g. SO₄²⁻.
ammonium ion
NH₄⁺, the most common positive polyatomic ion.
hydroxide ion
OH⁻; compounds with it, such as NaOH, are bases.
-ate / -ite
Endings for polyatomic ions with oxygen; the -ite ion has one fewer oxygen atom and the same charge.
brackets (in formulas)
Show that a subscript applies to a whole polyatomic ion, e.g. Ca(OH)₂.
More than two elements
Look at a box of baking soda. Its chemical name is sodium hydrogen carbonate, NaHCO₃. That formula contains four elements, so it is not a binary compound. Yet it is still ionic: it is made of Na⁺ ions and HCO₃⁻ ions. The hydrogen carbonate ion is a little team of atoms that stays together and carries a charge as one unit.
Teams like this are called polyatomic ions ("poly" means many). They are in fertilizers, drywall, chalk, bleach, Epsom salts and antacids, and they are on the list of things expectation C3.8 asks you to write formulas for.
What is a polyatomic ion?
A polyatomic ion is a group of two or more atoms that are covalently bonded together and that, as a group, have an overall charge.
- Inside the ion, the atoms share electrons (like a molecule).
- The whole group has gained or lost electrons, so it has a charge (like an ion).
- In a reaction or in a formula, the group usually stays together as one unit.
Almost all common polyatomic ions are negative. The main exception is the ammonium ion, NH₄⁺, which behaves like a metal ion.
| Charge | Ion | Formula | Where you might meet it |
|---|
| 1+ | ammonium | NH₄⁺ | fertilizers, some cleaners |
| 1− | hydroxide | OH⁻ | drain cleaner (sodium hydroxide) |
| 1− | nitrate / nitrite | NO₃⁻ / NO₂⁻ | fertilizers / cured meats |
| 1− | hydrogen carbonate (bicarbonate) | HCO₃⁻ | baking soda |
| 1− | chlorate | ClO₃⁻ | matches, fireworks |
| 1− | hypochlorite | ClO⁻ | bleach (sodium hypochlorite) |
| 1− | permanganate | MnO₄⁻ | water treatment |
| 1− | acetate | CH₃COO⁻ | vinegar (acetic acid) |
| 2− | sulfate / sulfite | SO₄²⁻ / SO₃²⁻ | Epsom salts, drywall / some wines |
| 2− | carbonate | CO₃²⁻ | limestone, chalk |
| 3− | phosphate | PO₄³⁻ | fertilizers, bones |
Patterns that make the list easier
- -ate and -ite: the -ite ion has one fewer oxygen atom than the -ate ion, with the same charge. Sulfate SO₄²⁻ → sulfite SO₃²⁻. Nitrate NO₃⁻ → nitrite NO₂⁻.
- "Hydrogen" in front: adding one H⁺ to an ion makes its charge 1 less negative. Carbonate CO₃²⁻ → hydrogen carbonate HCO₃⁻. Sulfate SO₄²⁻ → hydrogen sulfate HSO₄⁻. (The older name "bicarbonate" means the same as hydrogen carbonate.)
- Endings: a name ending in -ate or -ite almost always means a polyatomic ion with oxygen. An ending in -ide usually means a single-atom ion; hydroxide (OH⁻) is the famous exception.
Writing formulas with polyatomic ions
The rule is the same as in Grade 9: total positive charge = total negative charge. There is one new rule:
If you need more than one of a polyatomic ion, put it in brackets and write the subscript outside.
- Calcium hydroxide: Ca²⁺ needs two OH⁻ → Ca(OH)₂. Without brackets, CaOH₂ would mean one O and two H, which is wrong.
- Sodium hydroxide: Na⁺ needs only one OH⁻ → NaOH. No brackets needed for just one.
- Never change the subscripts inside a polyatomic ion. SO₄ is always SO₄. Changing it to SO₃ makes a different ion (sulfite).
You can still use the crisscross method. Treat the whole polyatomic ion as if it were one symbol, then reduce if you can.
Naming compounds with polyatomic ions
Name the positive ion first, then the negative ion. Polyatomic ions keep their own names; do not change the ending.
- NaNO₃: sodium nitrate. K₂CO₃: potassium carbonate. NH₄Cl: ammonium chloride.
- If the metal is multivalent, still use a Roman numeral: CuSO₄ is copper(II) sulfate; Fe(NO₃)₃ is iron(III) nitrate.
- Do not use prefixes. Ca(OH)₂ is calcium hydroxide, not "calcium dihydroxide".
To find a multivalent metal's charge, use the polyatomic ion's charge. In Fe(NO₃)₃, three nitrate ions give 3 × (1−) = 3−, so iron is 3+.
Worked exampleExample 1: Formula from a name
Write the formula for calcium phosphate.
- Calcium is in group 2: Ca²⁺. Phosphate is PO₄³⁻.
- Crisscross the charge numbers: Ca gets 3, PO₄ gets 2 → Ca₃(PO₄)₂. The phosphate goes in brackets because there are two of them.
- Check: 3 × (2+) = 6+ and 2 × (3−) = 6−. Total 0. ✔
Ca₃(PO₄)₂. (The hard mineral in your bones and teeth is a type of calcium phosphate.)
Worked exampleExample 2: Counting atoms
How many atoms of each element are in one formula unit of ammonium phosphate, (NH₄)₃PO₄?
- First check the formula: NH₄⁺ is 1+ and PO₄³⁻ is 3−, so three ammonium ions are needed. ✔
- Inside the brackets: 1 N and 4 H. The 3 outside multiplies both: 3 N and 12 H.
- Outside the brackets: 1 P and 4 O.
- Total: 3 + 12 + 1 + 4 = 20 atoms.
3 N, 12 H, 1 P, 4 O (20 atoms in total). Ammonium phosphate is used as a fertilizer.
Worked exampleExample 3: Names from formulas
Name (a) Cu(OH)₂, (b) NaClO, (c) Mg(HCO₃)₂.
- (a) Copper is multivalent. Two OH⁻ give 2−, so copper is 2+ → copper(II) hydroxide.
- (b) Na⁺ with ClO⁻ (hypochlorite) → sodium hypochlorite, the active chemical in household bleach.
- (c) Mg²⁺ with two HCO₃⁻ (hydrogen carbonate) → magnesium hydrogen carbonate.
(a) copper(II) hydroxide; (b) sodium hypochlorite; (c) magnesium hydrogen carbonate.
Hands-on activityCharge strips: build neutral compounds
You need: two colours of paper (or one colour and a marker to shade), a ruler, scissors (use them carefully), a pencil.
- Cut strips 2 cm tall. Make each strip 3 cm long for every unit of charge: a 1+ or 1− ion is 3 cm, a 2+ or 2− ion is 6 cm, a 3+ or 3− ion is 9 cm.
- Use one colour for positive ions and label them: Na⁺, K⁺, NH₄⁺ (3 cm); Ca²⁺, Mg²⁺, Cu²⁺ (6 cm); Al³⁺, Fe³⁺ (9 cm). Make two or three of each.
- Use the other colour for negative ions: OH⁻, NO₃⁻, HCO₃⁻ (3 cm); SO₄²⁻, CO₃²⁻ (6 cm); PO₄³⁻ (9 cm). Make three or four of each.
- Pick one positive and one negative ion. Lay positive strips in one row and negative strips in a row underneath, adding strips until the two rows are exactly the same length. That is a neutral compound.
- Write the formula (with brackets where needed) and the name. Then count the atoms.
Try these: magnesium nitrate, aluminium carbonate, ammonium sulfate, iron(III) hydroxide, calcium phosphate.
What to look for: Mg(NO₃)₂ (one 6 cm strip matches two 3 cm strips); Al₂(CO₃)₃ (two 9 cm = three 6 cm = 18 cm); (NH₄)₂SO₄; Fe(OH)₃; Ca₃(PO₄)₂ (three 6 cm = two 9 cm). The strip model shows why the subscripts are what they are: the total charge has to be zero.
Just for funWhat did the sulfate ion say after it lost an oxygen atom?
"I'm sulfite now, but I'm still just as negative about it." (Sulfite, SO₃²⁻, has the same 2− charge as sulfate.)
Your turn
1What is a polyatomic ion?
A group of atoms covalently bonded together that has an overall charge and acts as one unit.
2Write the formula for potassium nitrate.
K⁺ and NO₃⁻ → KNO₃ (charges already balance).
3Write the formula for magnesium hydroxide.
Mg²⁺ needs two OH⁻ → Mg(OH)₂.
Ammonium ion + nitrate ion → ammonium nitrate.
Sodium sulfate and sodium sulfite (sulfite has one fewer oxygen).
Two OH⁻ give 2−, so iron is 2+ → iron(II) hydroxide.
7How many oxygen atoms are in one formula unit of Al₂(SO₄)₃?
3 sulfate ions × 4 O = 12 oxygen atoms.
More practice: warm-up
1Give the formula and charge of the hydroxide ion.
2Give the formula and charge of the carbonate ion.
3Which common polyatomic ion is positive?
4What is the chemical name of baking soda?
Sodium hydrogen carbonate, NaHCO₃ (also called sodium bicarbonate).
5Why does Ca(OH)₂ need brackets but NaOH does not?
Calcium needs two hydroxide ions, and brackets show the 2 applies to the whole OH group. Sodium needs only one.
More practice: on your own
1Write the formula for aluminium nitrate.
Al³⁺ needs three NO₃⁻ → Al(NO₃)₃.
2Write the formula for ammonium carbonate.
Two NH₄⁺ balance one CO₃²⁻ → (NH₄)₂CO₃.
Carbonate is 2−, so copper is 2+ → copper(II) carbonate.
4Name KMnO₄ and give its composition.
Potassium permanganate: 1 K, 1 Mn and 4 O (one K⁺ ion and one MnO₄⁻ ion).
5A student writes calcium nitrate as CaNO₃₂. Explain the mistake and correct it.
The subscript 2 must apply to the whole nitrate ion, so it needs brackets: Ca(NO₃)₂. "NO₃₂" is meaningless and changes the ion.
Challenge
1Count all the atoms in one formula unit of Ca₃(PO₄)₂.
Ca: 3. Inside the brackets, 1 P and 4 O, times 2: 2 P and 8 O. Total 3 + 2 + 8 = 13 atoms.
2A compound has the formula Fe₂(SO₄)₃. Name it, and explain how you found the Roman numeral.
Three sulfate ions give 3 × (2−) = 6−. The two iron ions must share 6+, so each is 3+: iron(III) sulfate.
3Chromium forms Cr³⁺. Predict the formula of chromium(III) hydrogen carbonate and count the hydrogen atoms in one formula unit.
Cr³⁺ needs three HCO₃⁻ → Cr(HCO₃)₃. Each HCO₃⁻ has 1 H, so there are 3 hydrogen atoms.
Quiz
1. Which statement about a polyatomic ion is true?
2. What is the formula for calcium hydroxide?
4. How many oxygen atoms are in one formula unit of Mg(NO₃)₂?
5. What is the correct name for CuSO₄?
6. What is the formula for ammonium sulfate?
Big idea: expectation C3.8 asks students to write formulas for "simple compounds involving polyatomic ions (e.g., KNO₃, NaOH)" using a list of the most common polyatomic ions. Students are normally given that list on tests, so the focus is on using it correctly, not memorizing it.
Common difficulties:
- Forgetting brackets (writing CaOH₂ for Ca(OH)₂) or adding brackets when there is only one ion (Na(OH)).
- Changing the subscripts inside a polyatomic ion to "make the charges work". The ion's formula never changes; only the number of ions does.
- Mixing up -ate, -ite and -ide: nitrate NO₃⁻, nitrite NO₂⁻, nitride N³⁻ are three different ions.
- Counting atoms in brackets: the outside subscript multiplies everything inside.
How to help: the paper-strip activity makes charge balance visible; supervise scissors use. Look at labels together (baking soda, Epsom salts, bleach, fertilizer) and ask your teen to name the polyatomic ion in each. Keep products closed; reading the label is enough.