the Q-value

Every nuclear reaction or decay comes with a single number that says whether it releases energy or costs energy, and how much. That number is the Q-value. It is the nuclear equivalent of asking whether a chemical reaction is exothermic or endothermic, but computed from the change in rest mass, and it decides in one stroke whether a process can happen spontaneously at all.

The Q-value is the total rest-mass energy of the reactants minus that of the products: Q = (m_initial - m_final) c^2, summed over all particles on each side. Equivalently, it is the net kinetic energy released. A positive Q means the reaction is exothermic (mass converts to kinetic energy of the products) and can proceed spontaneously as far as energy is concerned; a negative Q means it is endothermic and requires an energy input, with a threshold kinetic energy needed to make it go. For a decay, Q > 0 is the necessary condition for the decay to be energetically allowed, and Q equals the kinetic energy shared among the products.

You compute Q from tabulated masses; using atomic (neutral) masses conveniently makes electron masses cancel in beta-minus decay but not in electron capture or beta-plus, where a careful accounting of 2 m_e c^2 matters. The crucial honesty is that Q > 0 is necessary but not sufficient for a decay to actually happen: alpha decay of many nuclei has Q > 0 yet is astronomically slow because of the Coulomb tunnelling barrier, and other transitions are throttled by angular-momentum selection rules. Energetics tells you if a door is open; it does not tell you how fast anything walks through.