Ionic Formula Factory

Why it matters

Imagine ions as puzzle pieces with specific numbers of tabs and slots. They only snap together when the charges balance perfectly, creating a neutral compound.

Predict first

If Magnesium (Mg²⁺) meets Chloride (Cl⁻), how many chloride ions are needed to balance one magnesium ion?

Magnesium has a +2 charge, so it needs two -1 chloride ions to reach a net charge of zero.

• One Cl⁻ ion
• Two Cl⁻ ions — correct
• Three Cl⁻ ions
• Half a Cl⁻ ion

What you do

Use the criss-cross method to build the neutral formulas for the given ion pairs. Drag the correct number of ions to balance the charge.

Check yourself

What is the correct formula for Aluminium Oxide?

Al³⁺ and O²⁻ cross their charges: Al takes 2, O takes 3, resulting in Al₂O₃.

The ions in magnesium oxide balance as the ratio Mg₂O₂. Why do we write its formula as MgO instead?

While Mg₂O₂ is charge-balanced, chemical convention requires the lowest whole-number ratio, which is 1:1.

Which pair of ions forms CaCl₂?

Calcium is in Group 2 (Ca²⁺) and Chlorine is in Group 17 (Cl⁻). Two Cl⁻ ions balance one Ca²⁺ ion.

• AlO
• Al2O3 — correct
• Al3O2
• AlO2
• To make the formula look shorter
• Ionic formulas represent the simplest whole-number ratio of ions — correct
• Because subscripts must always be reduced to 1 wherever possible
• Ca²⁺ and Cl⁻ — correct
• Ca⁺ and Cl⁻
• Ca²⁺ and Cl²⁻
• Ca⁺ and Cl²⁻