An ionic bond typically forms between a metal and a non-metal, where one atom donates electrons to another, resulting in charged ions.
Among the options provided:
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K (Potassium) is a metal and Ca (Calcium) is also a metal. They are both in the same category and are less likely to form an ionic bond together.
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C (Carbon) is a non-metal and O (Oxygen) is also a non-metal. They are more likely to form covalent bonds rather than ionic bonds.
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S (Sulfur) is a non-metal and Cl (Chlorine) is also a non-metal, which means they would more likely form covalent bonds.
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K (Potassium) is a metal and Cl (Chlorine) is a non-metal. This combination is likely to form an ionic bond, as potassium can donate an electron to chlorine.
Therefore, the pair most likely to form an ionic bond is K and Cl.