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Ionisation enthalpy
Concepts tested here
- Half-filled subshell stability
All Questions
2009 AIPMT 1 question
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Amongst the elements with following electronic configurations, which one of them may have the highest ionization energy?The configurations belong to Al ($3s^23p^1$), Si ($3s^23p^2$), P ($3s^23p^3$) and As ($4s^24p^3$).
Across a period the ionisation energy generally increases because the nuclear charge increases: Al < Si < P.
In addition, P has an exactly half-filled 3p subshell, which is extra stable, so removing an electron from it needs more energy.
As has the same half-filled p configuration but lies below P in group 15; its outer electron is farther from the nucleus, so its ionisation energy is lower than that of P.
Hence the element with configuration $[Ne]3s^23p^3$ has the highest ionisation energy.