lonization energy (Ei) is the amount of energy required to Based on position in the and electron configuration, arrange these elements in order of decreasing Ei1. remove an clectron from a neutral gaseous atom or gaseous ion. Rank the elements from highest to lowest ionization energy. To rank items as equivalent, overlap them. Electrons are attracted to the positively charged nucleus; therefore removing an electron requires energy. The process is endothermic, and so ionization energies have a positive value. The tirst ionization energy ( Ei1 ) is the energy associated with the removal of an electron from the neutral gasecus atom. The reaction is represented for the generallzed atom X as XX++e The amount of energy required to remove an electron is related to the effective nuclear charge and the stability of the electron configuration of the atom. It theretore shows periodic variation generally increasing from left to right in a period and from bottorn to top of a group. In general, metals have lower Ej1 values than nonmetals. Exceptions to this general trend from left to right occur when a completely filled s subshell or halt-filled subshell is encountered. These stable conligurations have larger than expected Eil values