Mg + O2 ➡️ MgO — we need to have the same emount of each element on both sides. On the left we have 1 Mg and on the right 1 Mg, so thats fine, but 2 O on the left and 1 O on the right, so we place a coefficient of 2 in front of the compound which is “lacking” O.
Mg + O2 ➡️ 2MgO
Now the O is okay, but we have 1 Mg and 2 Mg, so we need to put a 2 in front of the Mg thats “lacking”.
2M + O2 ➡️ 2MgO
Now it is balanced :)
