Which of the following elements has the largest first ionization energy?

• A. O
• B. Ne
• C. Li
• D. He

Answer: (D)

Verified step by step guidance

1. Understand that the first ionization energy is the energy required to remove one electron from a neutral atom in the gaseous state.
2. Recall the general trend in the periodic table: ionization energy increases across a period (left to right) due to increasing nuclear charge and decreases down a group due to increasing atomic radius and electron shielding.
3. Compare the given elements (O, Ne, Li) based on their positions in the periodic table: Li is in Group 1 (alkali metals), O is in Group 16, and Ne is a noble gas in Group 18, all in the second period.
4. Recognize that noble gases have the highest ionization energies in their respective periods because they have a full valence shell, making it harder to remove an electron.
5. Note that Helium (He), although not listed among the options, is a noble gas in the first period with a very small atomic radius and a full 1s shell, which results in the highest first ionization energy among all elements.