Ionization Energy

Consider the following experiment: Starting with hydrogen and proceeding through the periodic table, measure how much energy ot takes to pull an electron away from each atom. The resulting atom will be a cation and the energy it takes to pull the electron away is called the ionization energy.

If we plot these ionization energies versus atomic number the resulting graph below emerges.

Clearly, the most striking feature of this graph is that there is an inherent periodicity. How can this periodicity be explained? If we examine the electronic configurations of these elements we find that all elements with the lowest ionization energies -the alkali metals - have similar electronic configurations - [NG]ns2. Similarly, all the elements with the largest ionization energies - the noble gases- have filled s and p electronic configurations - [NG](nd10)ns2np6. The elements inbetween these two extremes also have similar electronic configurations.

Therefore, the periodicity of the data in the graph is a direct result of the underlying periodicity in the electronic configuration of the elements.