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Shown below are several options for the box notations of the ground state electron configuration of the following gas-phase species. Identify the correct electronic configuration.
(a)Select the reason that best explains why the first ionization energy decreases as atomic number increases for the alkali metals (Li, Na, K, Rb, Cs).
The higher the atomic number, the greater the force of attraction between the nuclear charge and an electron. As the force is inversely related to the energy required to liberate an electron from that force, the first ionization energy decreases as the force of attraction felt increases.
The outermost shell becomes occupied with a greater number of electrons as the atomic number increases resulting in greater electron-electron (a.k.a Born) repulsion, which in turn makes it easier to ionize one of these electrons.
All of these elements are in the same group, and have the same valence shell electron configuration. Even though the nuclear charge increases with atomic number, the outermost electron’s distance from the nucleus and the effectiveness with which inner electrons screen this greater nuclear charge both result in a net decrease in ionization energy.
The increase in ionization energy observed above is unrelated to the increasing atomic number, and instead has to do with the greater gravitational force of attraction to the heavier atomic nuclei.