The curve is the binding energy per nucleon B/A computed from the semi-empirical mass formula (aV = 15.8, aS = 18.3, aC = 0.714, aA = 23.2 MeV), for the most stable Z at each mass number A (the valley of stability). Move the slider to pick a nucleus: the readout shows the energy released if it splits into two halves (fission) or if two copies of it fuse.