Writing Formula of Ionic Compounds

Master Chemical Formulas! Before you can balance chemical equations or perform stoichiometry, you must know how to write correct chemical formulas. The fastest and most accurate way to predict the formula of an ionic compound is the Criss-Cross Method.

Predicting the Formula of Ionic Compounds

An ionic compound is formed when a metal (which loses electrons to become a positively charged cation) reacts with a non-metal (which gains electrons to become a negatively charged anion). Because the resulting compound must be electrically neutral, the total positive charge must exactly balance the total negative charge.

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Interactive Presentation: Formula Prediction

Click through the interactive presentation below to visually understand the rules, common ions, and step-by-step examples of predicting ionic formulas.

The 4 Steps of the Criss-Cross Method

If you need to quickly write a formula without drawing out electron transfers, the criss-cross method is your best tool. Follow these simple steps:

Step 1: Write the Symbols and Charges Write the symbol of the metal (cation) first, followed by the non-metal or polyatomic ion (anion). Include their respective charges as superscripts (e.g., Al³⁺ and O^2-).

Step 2: Criss-Cross the Numbers Take the numerical value of the cation's charge and make it the subscript for the anion. Take the numerical value of the anion's charge and make it the subscript for the cation. Drop the + and - signs.

Step 3: Simplify the Ratio (If Necessary) Ionic formulas must represent the lowest whole-number ratio of ions. If your subscripts are, for example, 2 and 4, divide them by their greatest common factor to get 1 and 2. (Note: We never write the subscript "1").

Step 4: Handle Polyatomic Ions Carefully If a subscript greater than 1 falls next to a polyatomic ion (like NO₃^- or SO₄^2-), you must put the entire polyatomic ion inside parentheses before adding the subscript.

Solved Examples

Sodium Chloride

Na⁺ and Cl^-

(Charges are 1:1, they cancel out)

NaCl

Aluminium Oxide

Al³⁺ and O^2-

(Criss-cross the 3 and 2)

Al₂O₃

Calcium Carbonate

Ca²⁺ and CO₃^2-

(Ratios simplify: 2:2 becomes 1:1)

CaCO₃

Frequently Asked Questions (FAQs)

Do I write the positive or negative ion first?

By universal chemical convention, the electropositive element (the cation/metal) is always written first, followed by the electronegative element (the anion/non-metal). For example, we write NaCl, never ClNa.

Why must ionic compounds be electrically neutral?

Ionic compounds form highly stable crystal lattice structures driven by electrostatic attraction. For the entire structure to be stable and not repel itself apart, the total positive charge must exactly cancel out the total negative charge, resulting in a net charge of zero.

When exactly do I need to use parentheses?

Parentheses are used only when a compound contains more than one of a specific polyatomic ion. For example, in Magnesium hydroxide, Mg(OH)₂, the parentheses indicate that there are two entire hydroxide (OH) groups for every one magnesium ion. If you wrote MgOH₂, it would incorrectly imply one oxygen and two hydrogens.

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