Answer: 2Mg (s) + O2 (g) ----> 2MgO (s)
Explanation:
Lets write the equation of the reaction.
Mg(s) + O2 (g) ----> MgO (s)
Counting the number of atoms for each element we have:
Left hand side: Mg =1 O = 2
Right hand side : Mg = 1, O =2
To balance this equation input "2" as coefficient for "MgO" on the right hand side and "2", as coefficient for "Mg" on the left hand side of the equation. Hence our balanced equation will be
2Mg (s) + O2 (g) -----> 2MgO (s)