1. The problem statement, all variables and given/known data 3H has more tightly bound than 3He. Why is it, then, that 3H beta-decays to 3He? 2. Relevant equations I also wonder what is the physical meaning of the value mass excess Δ(Z,A)≡[M(Z,A)-A]c2, or it is just for the convenience of calculations. 3. The attempt at a solution B/A of 3H is 2.8273 MeV while the B/A of 3He is 2.5727 MeV. We know that the requirement of beta decay is m(Z,A)>m(Z±1,A)+me, and m is the mass of nucleus because beta decay is the process of nucleus transition, not the process of atom transition. I guess the reason that 3H beta decays to 3He is just because the m(1,3)>m(2,3)+me, regardless of whether the daughter nucleus is more bound than mother nucleus. So beta decay has no direct relationship with the binding energy, which means there is also no directly relationship between beta stable line and binding energy, is it? Thank you for you time!