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Ar - Period 3, 3 quantum shells, proton number 18, electron number 18
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K - Period 4, 4 quantum shells, proton number 19, electron number 19 (the question is about K+ so that will reduce the quantum shells to 3, and electron number to 18)
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Ca - Period 4, 4 quantum shells, proton number 20, electron number 20 (the question is about Ca2+ so that will reduce the quantum shells to 3, and electron number to 18).
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