Q6 / 160 · AP EAMCET 2021 (23 Aug Shift 1) · Chemistry

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.

  • 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 $\mathrm{A}$.
  • Correct answer
    C
    $A$ is true but $R$ is false.
  • D
    $A$ is false but $R$ is true.

Solution

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.

Hence, the correct answer is option: 'C'.

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