The atom that would combine in a one-to-one ratio with calcium is oxygen.
How does atoms combine?
According to the Dalton's atomic theory, atoms combine in simple ratios to one another. As such, we can see that compounds are formed by the combination of atoms in a given ratio.
Now, the calcium atom which is divalent can only combine with a divalent atom in a one to one ratio. Since calcium is positively charged, this ion must be negatively charged to form an ionic bond.
Hence, the atom that would combine in a one-to-one ratio with calcium is oxygen.
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