Nuclear reactions in which the Q value is negative (endothermic reactions) imply that energy is absorbed in the process of the reaction. Is this to be interpreted as a case where the initial rest mass energy is less than the rest mass energy of the products of the reaction? If affirmative, it should be possible for an isolated proton to disintegrate as under: p -- n + e+ (positron) + Y(neutrino) However, the above is not possible because the mass of a proton is less than that of a neutron, and thus the reaction would imply a negative value of Q (mass energy on the left of the equation being less than that on the right). The reaction, otherwise, appears to be endothermic in nature because of the negative Q value. There, thus, appears to be a conflict in my understanding on this score. I would appreciate if the forum members could clarify the position to me, and to also provide me with one or two examples of endothermic reactions.