HELP on THIRD IONIZATION ENERGY eV (electron Volts)
The third ionization energy is the minimum energy required to remove a third electron from an atom that is already doubly ionized and when the element is in its gaseous state. Generally, it is the outermost electrons that are removed first. Third ionization energy varies periodically across the periodic table. In general it increases from left to right across a period, and decreases down a group. The increase across a period is as a result of a decrease in atomic size as more electrons are in the valence shell. This is because, the nearer the negatively charged electron to the positively charged nucleus, the more it is attracted, and the harder it is to wrench it from the atom. The decrease down a group is due to increasing atomic radius as more shells are added.
Note that helium cannot be triply ionized as it has only two electrons, and that lithium, when triply ionized, is fully ionised and is a naked nucleus. The third ionization energy is, in general, in proportion and about twice as great as the second ionization energy, with the wild exception of the alkali earth metals which are about five times greater. Suggest scattergram plot of second versus third ionization energy.
The third ionisation energy is highest for the alkali earth metals with beryllium having the highest at 154eV and radium the lowest (of the alkali earth metals) at 34eV. The alkali metals come a close second with lithium having the highest at 123eV and francium the lowest at 35eV. The lanthanide rare earth metals have the lowest third ionisation energies, with lanthanum having the lowest of all elements at 19eV.
The doubly ionised alkali earth metals, stripped of their two outer s-electrons, and now possessing a closed shell electronic structure similar to the noble gases, are very difficult to ionise further to the third degree.
