Which element has the highest first ionisation energy?
Answer and explanation
Correct answer: A. Helium
- Step 1: First ionisation energy increases across a period from left to right as effective nuclear charge rises, and decreases down a group due to increased electron shielding.
- Step 2: Helium resides at the top of Group 18 with a stable duet electron configuration (), where its electrons experience maximum nuclear attraction without any inner shielding shells.
- Step 3: Consequently, helium possesses the highest first ionisation energy () of all elements.
Answer
Helium.
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