To determine which ion has a stable valence electron configuration, we need to look for an ion that adopts a full outer electron shell, which is characteristic of the noble gases (group 18).
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Potassium (K): It typically loses one electron to form a K⁺ ion, achieving a configuration like argon (18 electrons). This configuration is stable.
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Fluorine (F): It typically gains one electron to form a F⁻ ion, achieving a configuration like neon (10 electrons). This is also stable.
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Magnesium (Mg): It typically loses two electrons to form a Mg²⁺ ion, which ends up with the same electron configuration as neon. That is stable as well.
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Sulfur (S): When sulfur gains two electrons, it becomes S²⁻, achieving a configuration similar to argon, which is considered stable.
Since all of these ions can achieve stable configurations by either gaining or losing electrons, the question may be asking for the one that is most common among them or perhaps the one that typically achieves a noble gas configuration through gaining rather than losing.
Answer: fluorine (F) with a 1− charge (F⁻) is a common choice since it gains an electron to achieve a stable noble gas configuration.