Hello, I've recently started studying Nuclear physics and would appreciate help from someone who could clarify my doubt. Given Problem: 19O -> 19F + e + v Calculate the Q-value in the given decay using following data: Atomic masses: 19O 19.003576u 19F 18.998403u The problem that I'm having is this: In the reaction I try to apply E=Δmc2 and I take Δm = mass(19O) - [mass(19F)+mass(electron)] But the solution of the problem, given in my textbook, ignores mass of electron. I don't get the idea of why we have to leave the electron's mass when the electron is there as the product of the given reaction on the right hand side. Book's solution: Q = [mass(19O)-mass(19F)]c2 Moreover, going through other such questions given in my textbook I have inferred that book's solution always takes Δm of the NUCLEUS of the atoms involved rather than take Δm of the whole reaction that is given. I think I'm missing out on something conceptual here. Any help will be appreciated.