The law of conservation of matter tells us that matter cannot be created or destroyed, it only changes. So in the reaction, the nitrogen and hydrogen atoms will not disappear but will be transformed and rearranged. Therefore, the number of atoms in the reactants must equal the number of atoms in the products.
Now, let's check how many atoms of each element we have on the reactant and product side.
Reactant side:
N=2
H=2
Product side:
N=1
H=3
We see that the number of atoms in the reactants is not equal to the number of atoms in the products. Therefore, the reaction does not obey the law of conservation of mass.
For it to obey the law we must balance the equation.
N2 + 3H2 ---> 2NH3
Reactant side:
N=2
H=6
Product side:
N=2
H=6