JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Classification of Elements and Periodicity

The CORRECT order of first ionisation enthalpy is:

(A) $M g < A l < S < P$
(B)Al $<$ Mg $<$ S $<$ P✓ Correct
(C)Mg $<$ S $<$ Al $<$ P
(D)Mg $<$ Al $<$ P $<$ S

Answer: (B)

Solution

First ionisation enthalpy rises across the third period as nuclear charge grows, but two configurations break the trend. In magnesium the electron is removed from a filled

3 s

subshell, whereas in aluminium it comes from a singly occupied

3 p

orbital that is higher in energy and screened by the

3 s

pair, so aluminium is easier to ionise than magnesium. Phosphorus has the stable half-filled

3 p^{3}

set, while sulphur must lose one of a pair of electrons sharing a single

3 p

orbital, which adds repulsion and lowers its value below that of phosphorus. The order is

Al < Mg < S < P

JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Single correct option

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JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Classification of Elements and Periodicity

Solution

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