**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, ^{6}H, ^{4}H, and ^{2}H, define
the shallow parabola on the left; the three isotopes with even numbers of neutrons,
^{5}H, ^{3}H, and ^{1}H, define the deeper parabola
on the right. The intercepts at Z/A = 0 yield values M/A = M(^{1}n)
+ 0.7 MeV for the parabola on the left and M/A = M(^{1}n) + 4.6 MeV
for the parabola on the right.