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Which of the following atoms has the lowest ionization potential?

Asked in JEE Main 2003 · Energy levels and ionisation

(1) ¹⁴₇N
(2) ¹³³₅₅Cs
(3) ⁴⁰₁₈Ar
(4) ¹⁶₈O

Answer: (2) ¹³³₅₅Cs

Step-by-step solution

Caesium is a large alkali metal; its single outer electron is far from the nucleus and well shielded.

So it is removed most easily.

Why the other options are wrong

(1) Nitrogen has a half-filled, tightly held shell.
(3) Argon is a noble gas with the highest of these.
(4) Oxygen's small size holds its electrons tightly.

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