Final answer:
Argon (Ar) has a complete valence electron shell, while the other elements mentioned do not.
Step-by-step explanation:
The element that has a complete valence electron shell is argon (Ar).
The valence electron shell is the outermost energy level of an atom. In the case of argon, it has a total of 8 valence electrons, which fills its second energy level and gives it a stable electron configuration.
Other elements mentioned, such as selenium (Se), oxygen (O), and fluorine (F), do not have a complete valence electron shell and are still seeking to gain or lose electrons to achieve a stable configuration.
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