Balancing Equations.

Aluminium belongs to group number 3. Therefore, it have 3valance electrons.while oxygen belongs to 6th group there fore it have. 6 valance electrons.
For to complete valancy oxygen needs two electron.
Now the balancing can be performed as;

4Al + 3O2———— 2Al2O3
EXPLANATION:

Aluminium have 3 valance electrons. So 4 aluminium have 12 electrons. Each oxygen can take 2 electrons so 6 Oxygen atoms can take 12 electrons.

So the number of loss and gain of electrons are equal and the number of elements on the reactant side is equal to the product side. Therefore, the equation is said to be a balance.

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