So far we have seen how quantum mechanics is applied to the simplest types of atoms/ions; those with only one electron. These situations are simple because there are only two particles to consider; one nucleus and one electron. In atoms/ions with more than one electron, things get more complicated. The presence of other electrons affects how each electron is attracted to the nucleus. The most important themes here are the concept of electron shielding and the effect of orbital penetration on electron shielding. These two ideas (electron shielding and orbital penetration) can explain periodic trends, and exceptions to these trends. It is critical that you have a solid knowledge of the radial part of the atomic orbital probability functions so that you can understand the discussion that follows.