Figure 6.  The six isotopes of hydrogen on a plot of mass per nucleon vs. charge density.  The three isotopes with odd numbers of neutrons, 6H, 4H, and 2H, define the shallow parabola on the left; the three isotopes with even numbers of neutrons, 5H, 3H, and 1H, define the deeper parabola on the right.  The intercepts at Z/A = 0 yield values M/A = M(1n) + 0.7 MeV for the parabola on the left and M/A = M(1n) + 4.6 MeV for the parabola on the right.