To determine which ion has a stable valence electron configuration, we need to consider the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a full outer shell of eight electrons, resembling the electron configuration of noble gases.
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Sulfur (S) with a 1- charge: Sulfur typically has six valence electrons. By gaining one electron, it achieves a total of seven valence electrons. This does not fulfill the octet rule (which requires eight).
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Magnesium (Mg) with a 1- charge: Magnesium has two valence electrons. If it were to gain a negative charge and gain an electron, it would have three valence electrons, which again does not fulfill the octet rule.
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Potassium (K) with a 1+ charge: Potassium has one valence electron. By losing this electron, it achieves a full outer shell (the next inner shell will contain eight electrons), resembling the electron configuration of noble gas argon.
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Fluorine (F) with a 1+ charge: Fluorine has seven valence electrons. If it were to lose an electron to become positively charged, it would then have six valence electrons, not fulfilling the octet rule.
Based on this analysis, the only ion with a stable valence electron configuration, achieving a full outer shell, is potassium (K) with a 1+ charge.