Which of the following atoms has the lowest ionization potential? (a) ¹⁴₇N (b) ¹³³₅₅Cs (c) ⁴⁰₁₈Ar (d) ¹⁶₈O

Answer: (b). Ionisation potential decreases down a group as the outer electron is farther from the nucleus and better shielded. Cs ( Z = 55) is an alkali metal with a single loosely bound 6s electron; N, O, Ar have much higher values. So the answer is (b) ¹³³₅₅Cs.

Which of the following atoms has the lowest ionization potential? | JEE Main 2003 (AIEEE)

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Energy of the Electron & Energy Levels

Which of the following atoms has the lowest ionization potential?

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Answer: (b)

Ionisation potential decreases down a group as the outer electron is farther from the nucleus and better shielded.
Cs ( $Z = 55$ ) is an alkali metal with a single loosely bound $6s$ electron; N, O, Ar have much higher values.
So the answer is (b) ${}^{133}_{55}\text{Cs}$ .

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