Polyatomic Ions and Their Valencies

Introduction

Sodium hydroxide, potassium nitrate, copper sulfate, calcium carbonate, ammonium chloride: each contains a small group of atoms that travels as a team. These groups are polyatomic ions . They behave much like single ions in compounds, with a fixed charge and a fixed valency. Learning a handful of them is one of the most useful things you can do in early chemistry, because they appear in acids, bases, salts, fertilisers and minerals.

Core explanation

What a polyatomic ion is. A polyatomic ion is a group of two or more atoms joined by covalent bonds that carries an overall electric charge. The atoms inside stay bonded together when the compound dissolves or reacts, so the group can be treated as one unit when writing formulae.

The five key ions.

Ion Formula Charge Valency Example compound --- --- --- --- --- Hydroxide OH⁻ 1− 1 sodium hydroxide, NaOH Nitrate NO₃⁻ 1− 1 potassium nitrate, KNO₃ Sulfate SO₄²⁻ 2− 2 copper(II) sulfate, CuSO₄ Carbonate CO₃²⁻ 2− 2 calcium carbonate, CaCO₃ Ammonium NH₄⁺ 1+ 1 ammonium chloride, NH₄Cl

Valency from charge. For any ion, the valency is the size of its charge. Hydroxide and nitrate have a charge of 1−, so their valency is 1. Sulfate and carbonate have a charge of 2−, so their valency is 2. Ammonium has a charge of 1+, so its valency is 1. A few others worth knowing are hydrogencarbonate (HCO₃⁻, valency 1) and phosphate (PO₄³⁻, valency 3).

Ammonium is the odd one out. Almost all common polyatomic ions are negative. Ammonium, NH₄⁺, is positive, so it takes the place of a metal ion in compounds: it is written and named first, as in ammonium chloride (NH₄Cl) and ammonium nitrate (NH₄NO₃). It forms when ammonia, NH₃, gains a hydrogen ion, H⁺.

Numbers inside the ion never change. The subscripts inside the ion belong to it: sulfate is always SO₄, never SO₃ or SO₅ (SO₃ would be a different ion, sulfite, or the molecule sulfur trioxide). When a compound needs more than one polyatomic ion, brackets are placed around the whole group, as in Ca(OH)₂ or Mg(NO₃)₂, rather than changing the numbers inside.

Spotting ions in formulae. Look for familiar groups: "OH" at the end of a metal formula means hydroxide; "NO₃" means nitrate; "SO₄" means sulfate; "CO₃" means carbonate; "NH₄" at the front means ammonium.

Step-by-step reasoning

To find the valency of a polyatomic ion in a compound such as Na₂CO₃:

1. Identify the known ion: sodium, Na⁺, valency 1. 2. Two sodium ions give a total of 2 units of positive charge. 3. There is one carbonate group, so it must carry 2 units of negative charge. 4. Carbonate's charge is 2−, so its valency is 2.

Visual explanation

Picture the sulfate ion as a small cluster: a yellow sulfur ball at the centre with four red oxygen balls around it, all wrapped in a bubble labelled "2−". The bubble moves as a single unit, and the charge belongs to the whole cluster, not to any one atom.

Real-world analogy

A polyatomic ion is like a band that always tours together. On a festival line-up the band has one name and one slot, even though it contains several musicians. In a formula, sulfate has one place and one charge, even though it contains five atoms.

Real-world example

Fertiliser bags often list ammonium nitrate, NH₄NO₃, and ammonium sulfate, (NH₄)₂SO₄. Both contain nitrogen that plants need for growth. The names reveal two polyatomic ions in each compound, one positive and one negative, and no metal at all.

Why?

Why do these groups stay together? The atoms inside are held by strong covalent bonds, while the whole group is attracted to oppositely charged ions by ionic bonding. When a salt dissolves, the ionic attractions are overcome but the covalent bonds inside the group are not, so the group moves as one ion.

Common misconception

"Sulfate and sulfur are the same thing." Sulfur is an element; sulfate, SO₄²⁻, is an ion containing one sulfur and four oxygen atoms with a 2− charge. Copper sulfate contains sulfate ions, not free sulfur.

Worked example

Question: Identify the polyatomic ions in NH₄NO₃ and state the valency of each.

Reasoning: The formula begins with NH₄, the ammonium ion, NH₄⁺, charge 1+. The remaining group, NO₃, is the nitrate ion, NO₃⁻, charge 1−. The charges balance with one of each.

Answer: Ammonium (valency 1) and nitrate (valency 1).

Quick check

1. What are the formula and charge of the carbonate ion? Answer: CO₃²⁻, with a charge of 2−.

Exam focus

You must know the formulae and charges of hydroxide, nitrate, sulfate, carbonate and ammonium by heart; they are rarely given in questions. Examiners also check that you use brackets around polyatomic ions when more than one is needed, and that you do not alter the numbers inside.

Advanced insight

Within a polyatomic ion the charge is spread over several atoms. In the carbonate ion, the three carbon–oxygen bonds are identical in length, because the extra electrons are shared across all three oxygen atoms. This sharing, called delocalisation, helps make ions such as carbonate, nitrate and sulfate stable.

Summary

Polyatomic ions are charged groups of atoms that act as single units. Hydroxide (OH⁻) and nitrate (NO₃⁻) have valency 1; sulfate (SO₄²⁻) and carbonate (CO₃²⁻) have valency 2; ammonium (NH₄⁺) is positive with valency 1 and replaces a metal ion. The atoms inside never change; brackets are used when more than one ion is needed.

Practice questions

1. Give the formula and charge of the nitrate ion. Answer: NO₃⁻, with a charge of 1−. 2. What is the valency of the sulfate ion? Answer: 2, because its charge is 2−. 3. Name the polyatomic ion in NaOH and give its valency. Answer: Hydroxide, OH⁻, valency 1. 4. Why is ammonium written first in the formula NH₄Cl? Answer: Ammonium is a positive ion, and positive ions are written and named first, as a metal ion would be. 5. Name all the ions present in potassium carbonate, K₂CO₃. Answer: Potassium ions (K⁺) and carbonate ions (CO₃²⁻), in a ratio of 2 : 1.