Which element has the highest first ionisation energy?
Answer and explanation
Correct answer: A. Helium
- Step 1: First ionisation energy is the minimum energy required to detach the outermost valence electron from an isolated gaseous neutral atom.
- Step 2: Helium possesses a closed electron configuration where two electrons occupy the innermost shell at a minimal distance from the nucleus without inner-shell screening.
- Step 3: Due to this exceptionally strong electrostatic nuclear attraction, helium exhibits the highest first ionisation energy () of all elements in the periodic table.
Answer
Helium.
Why not the others: Neon has a larger atomic radius () and greater electron shielding than helium, lowering its first ionization energy; Fluorine is a halogen with effective nuclear charge lower than the noble gases of its period; Hydrogen has only one proton, resulting in a lower nuclear charge than helium ( vs ).
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