Applet to Illustrate Nuclear Screening

The diagram above initially shows a atom of a noble gas as seen from a distance where you, the observer, are outside of any significant electron density. Sliding the scrollbar to the right produces the effect of moving closer and closer to the nucleus of the atom.

Initially, the nucleus is invisible, screened by the extra-nuclear electron cloud. As you begin to penetrate into the cloud, the nucleus becomes visible, and gradually more and more distinct. The screening effect diminishes because less and less of the electron cloud is between you and the nucleus.

If you were actually one of the electrons moving in a particular orbital, the size and energy of your orbital would depend on the degree to which the positive charge of the nucleus is screened by the other negatively charged electrons. That is, they would depend on the effective nuclear charge you "feel". An outer electron would move in the influence of a small effective nuclear chage, while a 1s electron would experience an effective nuclear charge approaching the true charge.