Ionization energy across a period
Which element has the highest first ionization energy?
Answer C: explanation for every choice
A
Chlorine is high but below argon, which has a full octet and a higher effective nuclear charge.
B
Silicon is in the middle of period 3.
C
Correct. Ionization energy rises across a period, and noble gases, with full shells, are highest in their period.
D
Sodium is lowest in period 3, since removing its lone 3s electron yields a stable configuration.
TakeawayIonization energy increases across a period and decreases down a group.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID cp39.