To determine which pair of elements is most likely to form an ionic bond, we need to identify elements with a significant difference in electronegativity, typically between a metal and a non-metal.
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Potassium (K) and fluorine (F): Potassium is a metal and fluorine is a non-metal. They have a high difference in electronegativity, making this pair likely to form an ionic bond.
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Aluminum (Al) and aluminum (Al): This involves two atoms of the same element (both are metals), which would not result in an ionic bond; rather, they would form a metallic bond.
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Sulfur (S) and oxygen (O): Both sulfur and oxygen are non-metals. While they can form covalent bonds, the bond would not be ionic.
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Calcium (Ca) and neon (Ne): Calcium is a metal, but neon is a noble gas and is mostly inert, meaning it does not readily bond with other elements.
Given this analysis, the pair most likely to form an ionic bond is potassium (K) and fluorine (F).