Ionization energy screening effect

The actual ionization energy of a helium atom is 3.94 X 10 J, about twice the fully screened value and about half the totally unscreened value. Screening is incomplete because both helium electrons occupy an extended region of space, so neither is completely effective at shielding the other from the +2 charge of the nucleus. 

C08-0003. The outer layers of the sun contain He atoms in various excited states. One excited state contains one 1 S electron and one 3 p electron. Based on the effectiveness of screening, estimate the ionization energy of the 3 p electron In this excited atom. 

Ionization energies deviate somewhat from smooth periodic behavior. These deviations can be attributed to screening effects and electron-electron repulsion. Aluminum, for example, has a smaller ionization energy than either of its neighbors in Row 3 ... 

As more electrons are introduced into the outer shell, the effective nuclear charge rises because electrons in the same shell are unable to screen the nuclear charge from one another very effectively, and the ionization energy therefore increases. This effect reaches its chmax when the shell is full, which occurs at neon for the L shell. 

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