Final answer:
The blank should be filled with the whole number '3' to balance the equation: 4 Al + 3 O2 → 2 Al₂O₃, ensuring that there are equal numbers of aluminum and oxygen atoms on both sides of the reaction.
Step-by-step explanation:
To fill in the blank with the correct number to balance the chemical equation, we need to ensure that there are an equal number of each type of atom on both sides of the equation. For the reaction 4 Al + __ O₂ → 2 Al₂O₃, we want to balance the aluminum (Al) atoms and the oxygen (O) atoms. There are 4 Al atoms on the left and 4 Al atoms on the right (since there are 2 units of Al₂O₃, and each contains 2 Al atoms). For oxygen, there are 3 oxygen molecules on the right (each Al₂O₃ has 3 O atoms, so 2 of them make 6 O atoms in total). To balance the equation with oxygen, we therefore need 3 O₂ molecules on the left side, since each O₂ molecule has 2 O atoms, and 3 O₂ molecules will give us a total of 6 O atoms. So the blank should be filled with the whole number '3' to balance the equation: 4 Al + 3 O₂ → 2 Al₂O₃.