Marcia is given an incomplete chemical equation that includes the number of nitrogen atoms present in the products of the reaction. how can she conclude that the reaction started with three nitrogen atoms?

If the products contain 3 nitrogen atoms, then so did the reactants since overall mass is conserved in a chemical reaction.

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.