To determine which pair of elements is most likely to form a molecule, we should consider their electronegativity and bonding characteristics.
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Calcium and oxygen: Calcium is a metal, and oxygen is a non-metal. They are likely to form an ionic bond (CaO), but not a molecule in the traditional sense (like covalent compounds).
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Magnesium and hydrogen: Magnesium is a metal and hydrogen can behave as both a metal and a non-metal. They can form a compound, but it is likely ionic (MgH₂) rather than covalent.
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Sodium and calcium: Both sodium and calcium are metals. They typically do not form covalent molecules together. Instead, they would likely form a metallic bond or not react at all in a way that produces a discrete molecule.
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Sulfur and bromine: Both sulfur and bromine are non-metals and can form a covalent bond, resulting in the formation of a molecular compound (like SBr₂).
Based on this analysis, the pair of elements most likely to form a molecule is:
Sulfur and bromine.