Examine the following set of ionization energy values for a certain element. How many valence electrons does an atom of the neutral element possess?
Ionization energy (Ei) is the amount of energy required to remove an electron from a neutral gaseous atom or gaseous ion. 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 first ionization energy (Ei) is the energy associated with the removal of an electron from the neutral gaseous atom. The reaction is represented for the generalized atom X as
X → X+ + 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 therefore shows periodic variation generally increasing from left to right in a period and from bottom to top of a group. In general, metals have lower Ei1 values than nonmetals. Exceptions to this general trend from left to right occur when a completely filled s subshell or half-filled p subshell is encountered. These stable configurations have larger than expected Ei1 values.
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