Formulas Containing Polyatomic Ions

Introduction

Some charged particles contain more than one atom. Nitrate, NO₃⁻, is an anion even though nitrogen and oxygen are covalently connected within it. When nitrate combines with a metal cation, the salt formula must balance the charge of each whole nitrate ion. Parentheses show when more than one identical polyatomic ion is present.

Core explanation

A polyatomic ion behaves as one charged unit for formula writing. Examples useful at this level include NH₄⁺ (ammonium), OH⁻ (hydroxide), NO₃⁻ (nitrate), CO₃²⁻ (carbonate) and SO₄²⁻ (sulfate). Their atoms are connected internally, commonly by covalent bonding, while the ion as a whole interacts electrostatically with counterions. The names and charges should be learned or supplied by a reference table; group-number rules for monatomic atoms do not reveal a polyatomic ion's net charge.

To make magnesium nitrate, combine Mg²⁺ with NO₃⁻. One Mg²⁺ contributes +2 and each nitrate contributes −1, so two nitrate ions are needed. Write Mg(NO₃)₂. The parentheses mean two copies of the complete NO₃ group: there are two nitrogen atoms and six oxygen atoms in a formula unit. MgNO₃₂ would not express that structure clearly. Do not put parentheses around one nitrate in NaNO₃, because a single group needs no external multiplier.

For calcium carbonate, Ca²⁺ and CO₃²⁻ balance in a 1:1 ratio, giving CaCO₃. For aluminium sulfate, Al³⁺ and SO₄²⁻ balance at total charge six: two Al³⁺ and three SO₄²⁻ produce Al₂(SO₄)₃. A whole-group subscript multiplies both sulfur and oxygen counts, but it does not alter the −2 charge of each sulfate ion. Charge labels belong to the constituent ions; subscripts in the final neutral formula are numbers of ions.

Ammonium illustrates a cation that is polyatomic. NH₄⁺ with Cl⁻ gives NH₄Cl. Two NH₄⁺ ions are needed to balance one SO₄²⁻, so ammonium sulfate is (NH₄)₂SO₄. Parentheses are required around NH₄ when the outer ₂ applies to the entire ammonium unit. In a solid ammonium salt, ammonium ions and anions form an extended ionic structure even though the N–H bonds inside each ammonium ion are covalent.

A common error is changing the internal formula of a known ion to achieve neutrality. Nitrate remains NO₃⁻; one does not turn it into “NO₆²⁻” when two nitrates are needed. Another error is to confuse oxidation states of atoms within nitrate with the net −1 charge of the entire ion. The formula-writing calculation uses the whole ion's charge. Later Lewis pages will examine how its internal electrons can be represented.

Step-by-step reasoning

1. Write the correct formula and net charge of each whole ion. 2. Find the smallest positive whole-number counts that give zero total charge. 3. Write the cation followed by the anion. 4. Put a polyatomic ion in parentheses only when its count exceeds one. 5. Check both overall charge and actual atom counts after applying subscripts.

Visual explanation

Draw a box around NO₃⁻ and label the box “−1 total.” Put two boxed copies beside one Mg²⁺, then write +2 + (−1) + (−1) = 0. Under the picture, show Mg(NO₃)₂. The box makes clear that the external subscript duplicates the whole ion, not just the last oxygen atom.

Real-world analogy

A sealed package may contain several objects but be counted as one package during shipping. Similarly, each nitrate ion has several atoms yet is counted as one charge-bearing unit while constructing the salt ratio. The comparison concerns counting; polyatomic ions are held by chemical bonds, not a physical wrapper.

Real-world example

Calcium carbonate, CaCO₃, is a major component of limestone and chalk. The formula says one Ca²⁺ for each CO₃²⁻ in the simple ionic accounting. Inside carbonate, carbon and oxygen are linked; the mineral's macroscopic properties arise from the crystal structure and interactions among many formula units.

Why?

Why is Al₂(SO₄)₃ neutral? Two Al³⁺ give +6. Three sulfate ions, each 2−, give −6. The parentheses ensure that the three applies to the entire sulfate group, giving three S and twelve O atoms per formula unit.

Common misconception

“The ₂ in Mg(NO₃)₂ turns nitrate into a 2− ion.” The ion remains NO₃⁻. The ₂ counts two separate nitrate ions. Distinguish an ion's charge from how many copies a neutral formula contains.

Worked example

Write the formula for ammonium phosphate using NH₄⁺ and PO₄³⁻. One phosphate ion contributes −3. Three ammonium ions contribute 3(+1) = +3, so the smallest neutral ratio is three ammonium ions to one phosphate ion. Write (NH₄)₃PO₄. Check the atom count: the formula has three nitrogen atoms, twelve hydrogen atoms, one phosphorus atom and four oxygen atoms. The charge check is +3 −3 = 0. The external subscript belongs outside ammonium's parentheses because it multiplies both N and H counts.

Quick check

1. Why are parentheses required in Ca(NO₃)₂ but not in NaNO₃? Answer: Calcium needs two whole nitrate ions for charge balance, whereas sodium needs only one.

Exam focus

Treat each named polyatomic ion as an intact charged unit. Show the total-charge arithmetic, reduce to the smallest ratio and use parentheses only for a repeated group. Count atoms only after the final formula is correctly written.

Advanced insight

Calling a salt “ionic” describes the interaction between its charged units, not every bond inside those units. A polyatomic ion can contain delocalised covalent bonding and still participate in an ionic lattice. This is a concrete example of why one bonding label rarely describes all length scales of a compound.

Summary

Polyatomic ions carry a net charge as whole groups. Salt formulas balance those charges in smallest integer ratios, and parentheses show when a group repeats. Internal covalent bonds and external ionic interactions can occur within the same substance.

Practice questions

1. What is the formula of calcium hydroxide from Ca²⁺ and OH⁻? Answer: Ca(OH)₂; two hydroxide ions balance one calcium ion. 2. Why is NaNO₃ written without parentheses? Answer: Only one nitrate ion occurs per sodium ion in the simplest ratio. 3. How many oxygen atoms are represented in Al₂(SO₄)₃? Answer: Twelve, because three sulfate ions each contain four oxygen atoms. 4. What charge does each ammonium ion have in (NH₄)₂SO₄? Answer: Each NH₄⁺ has +1; two balance one sulfate ion's −2.