Formula Builder
Use criss-cross method to write formulas
What this lesson covers
Why it matters
Imagine radicals as puzzle pieces with specific numbers of tabs and slots. A formula is only stable when every tab finds a slot, leaving no loose ends.
Predict first
If Calcium (Ca) has a valency of 2 and Chlorine (Cl) has a valency of 1, how many Chlorine atoms are needed to balance one Calcium atom?
Calcium needs to lose 2 electrons, while each Chlorine can only accept 1. You need two Chlorines to catch Calcium's two electrons.
- One (CaCl)
- Two (CaCl₂) — correct
- Three (CaCl₃)
What you do
Cross each pair's valencies to build NaCl, CaCl2, MgO, then Al2O3. Swap the charges into subscripts and the formula falls out.
Check yourself
Why is the formula for Magnesium Oxide MgO and not Mg₂O₂?
Even though 2 Mg²⁺ and 2 O²⁻ balance, we simplify the ratio 2:2 to 1:1, writing MgO.
Aluminium has a valency of 3 and Sulphate has a valency of 2. What is the correct formula?
Cross the valencies: Al(3) and SO₄(2) become Al₂(SO₄)₃. The brackets are needed because Sulphate is a polyatomic radical.
- Because Mg₂O₂ is unstable
- Chemical formulas show the simplest whole-number ratio of atoms — correct
- Oxygen is heavier than Magnesium
- AlSO₄
- Al₂(SO₄)₃ — correct
- Al₃(SO₄)₂