The first ionization energy of magnesium is smaller, as compared to that of elements X and Y, but higher than that of Z. The elements X, Y and Z, respectively, are:

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Strategy:
 Ionization enthalpy increases toward the top-right of the table. Compare the position of Magnesium (Gr 2, Per 3) with other options.

Step 1: Check trends for Mg
-  (Group 1 < Group 2).
-  (contradicts anomaly).
-  and .

Step 2: Conclusion
For Magnesium to be smaller than X and Y but higher than Z:
- Z must be Sodium (Na) (Group 1 element).
- X and Y must be elements to the right, such as noble gases (Ar) or halogens (Cl).

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