2Mg + O2 —> 2MgO
It’s quite simple, really, just add up the total amount of each element on both sides. If there is a difference in the amount of atoms on either side, that equation isn’t balanced.
Take the first equation, Al + 3F2 —> AlF3.
There is one Al atom on each side, so it is properly balanced, but with fluorine, there are 6 on the reactant side and only 3 on the product side. So the entire equation is not properly balanced.
If it wasn’t clear, the 6 fluorine atoms on the products side came from the multiplication of the coefficient on F (3) and the subscript (2). The coefficient 3 basically means there are three molecules of F2.
