Which of the following representative elements requires the greatest energy to remove its first valence electron?
Answer and explanation
Correct answer: B. Helium (He)
- Step 1: Review periodic trends in ionization energy: Ionization energy increases across a period and decreases down a group.
- Step 2: Identify the noble gas in period 1: Helium has a full valence shell closest to the nucleus with maximal effective nuclear charge and minimal shielding.
- Step 3: Confirm: Helium has the highest first ionization energy () of all known chemical elements.
Answer
Helium (He).
Why not the others: Sodium and Magnesium are alkali/alkaline earth metals with low ionization energies; Phosphorus has a moderately high ionization energy in period 3 but far below Helium.
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