Balance the chemical equation: □ Al + □ O₂ → □ Al₂O₃

What you are trying to match

A chemical equation is balanced when each element has the same number of atoms on the reactant and product sides.

Balance aluminum first

$Al_2O_3$ contains 2 Al atoms per formula unit, but the reactant side has $Al$ atoms one at a time. Put a 2 in front of $Al_2O_3$ to make the product side have 4 aluminum atoms:

$$Al + O_2 \rightarrow 2Al_2O_3$$

Now balance aluminum by putting a 4 in front of $Al$:

$$4Al + O_2 \rightarrow 2Al_2O_3$$

Balance oxygen using $O_2$

With $2Al_2O_3$, the product side has $2 \times 3 = 6$ oxygen atoms. Since oxygen comes as $O_2$ (pairs), you need 3 molecules of $O_2$ to get 6 oxygen atoms:

$$4Al + 3O_2 \rightarrow 2Al_2O_3$$

Quick check

• Aluminum: left $4$, right $2 \times 2 = 4$
• Oxygen: left $3 \times 2 = 6$, right $2 \times 3 = 6$

So the equation is balanced.