Which of the following has the highest ionization energy? 

• A. Ar
• B. Na⁺
• C. Na
• D. Mg
• E. Kr

Answer: (B)

Verified step by step guidance

1. Understand the concept of ionization energy: Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. Generally, ionization energy increases across a period from left to right and decreases down a group in the periodic table.
2. Identify the position of each element or ion in the periodic table: Ar (argon) and Kr (krypton) are noble gases, Na (sodium) and Na+ (sodium ion) are in Group 1, and Mg (magnesium) is in Group 2.
3. Consider the electron configuration: Noble gases like Ar and Kr have full outer electron shells, making them very stable and requiring more energy to remove an electron. Na+ has a full outer shell due to losing one electron, making it more stable than Na.
4. Compare the ionization energies: Noble gases typically have high ionization energies due to their full valence shells. Na+ has a higher ionization energy than Na because it is already positively charged and losing another electron would require more energy.
5. Determine the highest ionization energy: Among the given options, Ar and Kr are noble gases with high ionization energies, but Ar is higher in the periodic table than Kr, generally indicating a higher ionization energy. Na+ also has a high ionization energy due to its positive charge and full outer shell.