Correct answer: 2 NaN_3(s) →  2 Na_(s) +  3 N_2(g)

Why?:

The Law of Conservation of Mass states that the total mass of the reactants is equal to the total mass of the products. This also means that chemical equations should be balanced. In balancing an equation, the number of atoms appearing on the reactant side must equal the number of atoms appearing on the product side. If there is a total of three sodium atoms on the reactant side, then there should also be a total of three sodium atoms on the product side of the equation.

Answering this question:

On the reactant side, there is 1 sodium atom and 3 nitrogen atoms. On the product side, there is 1 sodium atom and 2 nitrogen atoms. To balance the nitrogen atoms, we must look for a factor that is a multiple of 2 and 3. Since 6 is the lowest multiple of the numbers 2 and 3, we will use 6. On both sides of the reaction, there should be 6 nitrogen atoms. So we write a coefficient of 2 for NaN₃ and 3 for N₂. This way, the number of nitrogen atoms on both sides of the reaction is equal. However, we are still left with the sodium atom to balance. Since there are now 2 atoms of sodium on the reactant side and only 1 on the product side, then we should write a coefficient of 2 for Na on the product side. With this, the number atoms on the reactant side are balanced with the number of atoms on the product side.

So the correct answer is 2 NaN_3(s) → 2 Na_(s) + 3 N_2(g)