Formula Jigsaw Puzzle
Use criss-cross method to write compound formulas
Setting up the bench…
What this lesson covers
Why it matters
Imagine atoms as puzzle pieces with specific numbers of tabs and slots. They only click together when every tab finds a matching slot.
Predict first
Why do we write Magnesium Oxide as MgO and not MgO₂?
Chemical formulas are neutral. The total positive charge must equal the total negative charge.
- Because oxygen is heavier than magnesium
- Because Mg²⁺ and O²⁻ valencies cancel out in a 1:1 ratio — correct
- Because there is only one oxygen atom in the universe
What you do
Fit the pieces together. Ensure the total positive valency equals the total negative valency for each target.
Check yourself
What is the correct formula for Aluminium Oxide?
Al has valency 3, O has valency 2. Cross-multiply to balance: Al₂O₃.
In Calcium Chloride (CaCl₂), why are there two chlorine atoms?
Ca²⁺ requires two Cl⁻ ions to achieve electrical neutrality.
- AlO
- Al₂O₃ — correct
- Al₃O₂
- AlO₂
- Chlorine is diatomic in compounds
- Calcium needs two Cl⁻ ions to balance its +2 charge — correct
- Chlorine is twice as heavy as calcium