For $He^+$, the ionisation energy from its ground state is
A$27.2\,eV$
B$54.4\,eV$
C$13.6\,eV$
D$6.8\,eV$
Answer & Solution
Correct answer: B. $54.4\,eV$
$IE = 13.6 \cdot Z^2 = 13.6 \cdot 4 = 54.4\,eV$ for $He^+$ ($Z=2, n=1$).
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