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Question: Answered & Verified by Expert
Assertion (A)
The first ionisation energy of Be is greater than that of $\mathrm{B}$.
Reason (R) $2 p$ orbital has lower energy than $2 s$ orbital.
ChemistryClassification of Elements and Periodicity in PropertiesAP EAMCETAP EAMCET 2021 (23 Aug Shift 1)
Options:
  • A Both $\mathrm{A}$ and $\mathrm{R}$ are true and $\mathrm{R}$ is a correct explanation for $\mathrm{A}$.
  • B Both $\mathrm{A}$ and $\mathrm{R}$ are true but $\mathrm{R}$ is not a correct explanation for $\mathrm{A}$.
  • C $A$ is true but $R$ is false.
  • D $A$ is false but $R$ is true.
Solution:
1387 Upvotes Verified Answer
The correct answer is: $A$ is true but $R$ is false.
The first IE of Be is greater than that of B because Be has stable complete electronic configuration $\left(1 s^2 2 s^2\right)$, which require more energy to remove the first electron from its valence shell, whereas, B has electronic configuration $1 s^2 2 s^2 3 s^1$ which require lesser energy than that of Be. In atoms with more than one electron, $2 s$-orbital is lower in energy than $2 p$-orbital.
So, $\mathrm{A}$ is true, but $\mathrm{R}$ is false.

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