Assertion (A) Boron has a smaller first ionisation enthalpy than beryllium. Reason (R) The penetration of a $2 s$-electron to the nucleus is more than the $2 p$-electron hence, $2 p$-electron is more shielded by the inner core of electrons than $2 \mathrm{~s}$-electrons.

Question

Assertion (A) Boron has a smaller first ionisation enthalpy than beryllium. Reason (R) The penetration of a $2 s$-electron to the nucleus is more than the $2 p$-electron hence, $2 p$-electron is more shielded by the inner core of electrons than $2 \mathrm{~s}$-electrons., Assertion and Reason both are correct statements but Reason is not correct explanation for Assertion., Assertion is correct statement but Reason is wrong statement., Assertion and Reason both are correct statements and Reason is correct explanation for Assertion., Assertion and Reason both are wrong statements.

MARKS App · Accepted Answer

Assertion (A) Electronic configuration of boron $(Z=5)$ is [He] $2 s^2 2 p^1$. So, in first ionisation ( $1 \mathrm{IE}_1$ or $\Delta_i H_1$ ) removal will take place from unpaired $p^1$-electron. Whereas that of Be will be from paired $2 s^2$-electrons which requires more energy. Electronic configuration of $\mathrm{Be}(Z=4):[\mathrm{He}] 2 s^2$ So, the Assertion is a correct statement. Reason (R) $s$-orbital is being symmetrical in shape (spherical), it shields nuclear force (nuclear charge) strongly. So, $2 p^1$-electron of $B$ is experiences lesser nuclear attractive force for ionisation. As a result, $\mathrm{IE}_1 \text { or } \Delta_i H_1: \mathrm{B} < \mathrm{Be}$ So, the Reason is correct explanation for Assertion.

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