The atomic number of the element from the following with lowest $1^{\text {st }}$ ionisation enthalpy is:

JEE Main · 2025 · 08 Apr 2025 · Shift 2 · Classification of Elements and Periodicity

The atomic number of the element from the following with lowest

1^{st}

ionisation enthalpy is:

(A)32
(B)35
(C)87✓ Correct
(D)19

Answer: (C)

Solution

The four atomic numbers correspond to germanium (32), bromine (35), francium (87) and potassium (19). First ionisation enthalpy falls on moving down a group, because the valence electron occupies a larger, better-shielded orbital held more loosely by the nucleus. Francium lies at the foot of Group 1, so its

7 s^{1}

electron is the farthest out and the most easily removed of the set. The element of atomic number 87, francium, has the lowest first ionisation enthalpy.

JEE Main · 2025 · 08 Apr 2025 · Shift 2 · Single correct option

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