Of the following atoms, which has the largest third ionization energy? 

• A. Al
• B. Ca
• C. K
• D. Ba
• E. Cs

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 in the gaseous state. The third ionization energy refers to the energy needed to remove the third electron after the first two have been removed.
2. Consider the electron configuration of each element: Determine the electron configuration for Al, Ca, K, Ba, and Cs. This will help identify which electron is being removed during the third ionization.
3. Analyze the electron configuration changes: For each element, consider the electron configuration after the removal of the first and second electrons. The third ionization energy will involve removing an electron from a more stable electron configuration, often from a filled or half-filled shell.
4. Evaluate the stability of the resulting ion: The third ionization energy is typically higher if the electron being removed is from a stable configuration, such as a filled or half-filled shell. Compare the stability of the resulting ions for each element.
5. Compare the third ionization energies: Based on the electron configurations and the stability of the resulting ions, determine which element has the highest third ionization energy. Elements with a stable electron configuration after the removal of the third electron will have higher ionization energies.