Because the other terms in the Hamiltonian essentially describe two hydrogen atoms except for the
nuclear charge of 2, we could simply neglect the repulsion between the two electrons to obtain
an equation that can be solved exactly. In other words, we have “ approximated ” the system as two
hydrogen atoms, which means that the binding energy of an electron to the helium nucleus should
be 27.2 eV, twice the value of 13.6 eV for the hydrogen atom. However, the actual value for the fi rst
ionization potential of helium is 24.6 eV because of the repulsion between the two electrons. Clearly,
the approximate wave equation does not lead to a correct value for the binding energy of the electrons
in a helium atom. This is equivalent to saying that an electron in a helium atom does not experience
the effect of being attracted by a nucleus having a 2 charge, but an attraction which is less than that
value because of the repulsion between electrons. If this approach is taken, it turns out that the effec-
tive nuclear charge is 27/16 1.688 instead of exactly 2.