First ionization energy is the energy required to remove the highest energy electron from an atom. The second ionization energy is the energy required to remove an electron from a unipositive ion of the element.
The following graph gives a 3-D representation of first ionization energy for the most of the elements. Concentrate on the overall trends in the periods and groups, then the exceptions in the pattern will be considered.
More on Ionization Energy [local].
Three factors control ionization energy:
There is a connection between the size of an atom and its ionization energy; smaller atoms have electrons closer to the nucleus, and thus have higher ionization energies.
Elements that will tend to easily form +1 ions are those with a single valence electron (the alkali metals). The trend in the second ionization energy is similar to the first ionization energy shifted one element to the right. The highest second ionization energies now occur in Group I elements with the Group II showing the lowest. The 3-D table below shows the second ionization energies of the elements. Values for italicized elements are not known.
In a similar fashion the group with the highest third ionization energy is Group IIA, and Group IIIA has the highest fourth ionization energy. This pattern helps explain why the Group II elements form +2 ions commonly and Group III form +3 ions.
Summary of trends in the representative elements:
Group I metals lose elctrons easily. They have the lowest ionization energies in their period. They will not form a +2 ion easily; their second ionization energies are high. These elements have one electron in their highest energy level; once it is lost, any other electrons removed would need to be taken from a filled lower level.
The Noble Gases have the highest ionization energy on each period, followed by the halogen elements. These atoms are also the smallest in their respective period, since the electrons are closer to the nucleus and have a stronger attraction for its positive charge.
The following graph shows the variation in first ionization energy through radon. One of the first features that should stand out is the cyclic nature of the property. The next aspect to concentrate on is the connection between electron configuration and the changes in the ionization energy. The change in subshells and the connection to Hund's rule should be obvious in the examination of the first three periods. It is also obvious that the extremes in variation diminish as the atoms get larger, with more and more energy levels.
