Writing Formulae of Salts

Introduction

Knowing a salt's name is only half the story; chemists also need its formula to write equations and calculate masses. Why is sodium chloride NaCl but calcium chloride CaCl₂? Why is aluminium sulfate Al₂(SO₄)₃? The answer is always the same: a salt has no overall charge, so its positive and negative charges must cancel exactly. With a short table of ion charges and one simple rule, you can write the formula of any common salt.

Core explanation

The rule. In a salt, the total positive charge equals the total negative charge. The formula shows the simplest ratio of ions that makes the overall charge zero.

Common positive ions.

Charge Ions --- --- 1+ Na⁺, K⁺, Li⁺, Ag⁺, NH₄⁺ 2+ Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺, Fe²⁺, Ba²⁺ 3+ Al³⁺, Fe³⁺

For Group 1, 2 and 3 metals, the charge equals the group number. For transition metals, the Roman numeral in the name gives the charge.

Common negative ions.

Charge Ions --- --- 1− Cl⁻, Br⁻, NO₃⁻, OH⁻, CH₃COO⁻, HCO₃⁻ 2− SO₄²⁻, CO₃²⁻, O²⁻ 3− PO₄³⁻

The charge on an acid's anion equals the number of hydrogens the acid loses: HCl → Cl⁻, H₂SO₄ → SO₄²⁻, H₃PO₄ → PO₄³⁻.

Balancing examples.

- Na⁺ and Cl⁻: one of each cancels, giving NaCl. - Ca²⁺ and Cl⁻: two Cl⁻ are needed for each Ca²⁺, giving CaCl₂. - Na⁺ and SO₄²⁻: two Na⁺ for each SO₄²⁻, giving Na₂SO₄. - Al³⁺ and SO₄²⁻: the lowest common multiple of 3 and 2 is 6, so two Al³⁺ (6+) and three SO₄²⁻ (6−), giving Al₂(SO₄)₃.

Brackets for compound ions. A compound ion is a group of atoms that stays together, such as NO₃⁻ or SO₄²⁻. When more than one is needed, put it in brackets with the subscript outside: Mg(NO₃)₂, not MgNO₃₂; (NH₄)₂SO₄, not NH₄₂SO₄. If only one compound ion is present, no brackets are needed: CaCO₃, NaNO₃.

The crossover shortcut. A quick method is to swap the charge numbers and use them as subscripts: Al³⁺ and O²⁻ give Al₂O₃. Always simplify afterwards: Mg²⁺ and O²⁻ give Mg₂O₂, which simplifies to MgO. Formulae of ionic compounds are always the simplest ratio.

Charges are not written in the formula. The final formula shows only symbols and subscripts, such as CuSO₄, not Cu²⁺SO₄²⁻.

Step-by-step reasoning

To write a salt formula from its name:

1. Write the symbol and charge of the positive ion. 2. Write the formula and charge of the negative ion. 3. Find the numbers of each needed to make total charge zero. 4. Write the formula, using brackets for more than one compound ion. 5. Check: total positive charge = total negative charge.

Visual explanation

Picture ions as jigsaw pieces: a 2+ ion has two notches, a 1− ion one tab. Ca²⁺ needs two Cl⁻ pieces to fill its notches, giving a complete, flat piece with no gaps — CaCl₂. Al³⁺ and SO₄²⁻ only fit perfectly when two aluminium pieces meet three sulfate pieces.

Real-world analogy

Balancing charges is like paying an exact amount with coins. If one pile may contain only 3p coins and another only 2p coins, and both piles must be worth the same, the smallest match is two 3p coins against three 2p coins, 6p each — just like two Al³⁺ ions balancing three SO₄²⁻ ions.

Real-world example

Aluminium sulfate, Al₂(SO₄)₃, is used in water treatment to make tiny suspended particles clump together so they can be removed. Water engineers need its correct formula to calculate how much to add to each volume of water.

Why?

Why must the charges cancel? Opposite charges attract, and a lattice with extra positive or negative charge would repel further ions of the same sign. Stable solid salts therefore form in the exact ratio that makes them electrically neutral.

Common misconception

"The formula of calcium nitrate is CaNO₃₂." The subscript 2 applies to the whole nitrate ion, so brackets are needed: Ca(NO₃)₂, meaning one calcium, two nitrogen and six oxygen atoms.

Worked example

Question: Write the formula of iron(III) sulfate.

Reasoning: Iron(III) is Fe³⁺ and sulfate is SO₄²⁻. The lowest common multiple of 3 and 2 is 6, so two Fe³⁺ (6+) balance three SO₄²⁻ (6−).

Answer: Fe₂(SO₄)₃

Quick check

1. Write the formula of magnesium chloride. Answer: MgCl₂, because one Mg²⁺ ion needs two Cl⁻ ions.

Exam focus

Learn the charges of the common ions, especially sulfate (2−), nitrate (1−), carbonate (2−), hydroxide (1−) and ammonium (1+). Use brackets correctly; many marks are lost by writing MgNO₃₂ or NH₄₂SO₄. Always check that charges balance.

Advanced insight

Some salts include water molecules in their crystal lattice, written after a dot, such as CuSO₄·5H₂O. The water is part of the crystal structure but not part of the charge balance. Formulae of ionic compounds give ratios, not molecules, which is why we speak of "formula units" and relative formula mass.

Summary

Salt formulae are built by balancing ionic charges so the total is zero. Use the charges of the metal (group number or Roman numeral) and the acid anion (number of hydrogens lost). Put compound ions in brackets when more than one is needed, simplify to the lowest ratio, and leave charges out of the final formula.

Practice questions

1. Write the formula of potassium sulfate. Answer: K₂SO₄, since two K⁺ ions balance one SO₄²⁻ ion. 2. Write the formula of ammonium carbonate. Answer: (NH₄)₂CO₃, since two NH₄⁺ ions balance one CO₃²⁻ ion. 3. Write the formula of aluminium nitrate. Answer: Al(NO₃)₃, since three NO₃⁻ ions balance one Al³⁺ ion. 4. Write the formula of calcium phosphate. Answer: Ca₃(PO₄)₂, because three Ca²⁺ (6+) balance two PO₄³⁻ (6−).