Up to Neon

For all atoms beyond helium, there is a core consisting of the nucleus and the two tightly bound E₁ electrons. As the charge on the nucleus increases, the size of the E₁ patterns shrink, and are penetrated less and less by
the outer electrons. We can think of this helium core as acting as the effective nucleus for the larger atoms.
As we go from boron to neon, the charge on the helium core increases from 3e to 8e as the E₂ , = 1 patterns fill up. This increase in the charge of the core causes a more or less gradual increase in the binding energy, from 8.30 eV up to 21.56 eV. The one exception is the slight drop in binding energy as we go from nitrogen to oxygen. The arguments we have made so far are not detailed enough to explain this drop.