The mass number of the reactant, U, is 236. Because the actual mass is 236.045 563 Da, its mass excess is + 0.045 563 Da. Calculated in the same manner, the respective mass excesses for the products, Kr, Ba, and three neutrons, are −0.073 843 Da, −0.085 588 Da and 3 × 0.008 665 Da = + 0.025 994 Da, respectively, for a total mass excess of −0.133 437 Da. The difference between the mass excess of the reactants and that of the products is 0.179 000 Da, which shows that the mass excess of the products is less than that of the reactants, and so the fission can occur – a calculation which could have also been done with only the masses of the reactants.