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 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 after losing one electron, making it more stable than Na.
4. Compare the ionization energies: Generally, noble gases have the highest ionization energies due to their full valence shells. Among the given options, Ar and Kr are noble gases, but Ar is higher in the periodic table, which typically means it has a higher ionization energy than Kr.
5. Conclude based on periodic trends: Since Ar is a noble gas and is higher in the periodic table than Kr, it has the highest ionization energy among the given options.