Question

Assertion (A) : Boron has a smaller first ionisation enthalpy than beryllium.

Reason $\left(R\right):$ The penetration of a $2s$ electron to the nucleus is more than the $2p$ electron hence $2p$ electron is more shielded by the inner core of electrons than the $2s$ electrons.

• A. Assertion and reason both are wrong statements.
• B. Assertion is a correct statement but reason is wrong.
• C. Assertion and reason both are correct statements and reason is the correct explanation for assertion
• D. Assertion and reason both are correct statements but reason is not correct explanation for assertion

Answer 1

The correct option is C

Assertion and reason both are correct statements and reason is the correct explanation for assertion

Analysing the assertion

Boron has a smaller first ionisation enthalpy than beryllium.

$Be=1s^{2}2s^2$

$B=1s^{2}2s^{2}2p^1$

So, from the electronic configuration we can say Be have more stable electronic configuration than $B,$ as, outer orbital of Be is completely filled, so it will require more
energy to remove any electron from its valence shell.

Hence, assertion is correct.

Analysing the reason

The penetration of a $2s$ electron to the nucleus is more than the $2p$ electron, hence, electron is more shielded by the inner core of electrons that the $2s$ electrons.

Penetration effect - $s>p>d>f$

The reason is correct as $2s$ electrons are present more closer to the nucleus in comparison to $2p$ electrons.

Due to which, for $2p$ electrons the shielding effect will be higher (by the core electrons) and it will compensate more the nuclear charge and thus higher energy will be required to remove an electron from $2s$ than $2p$

Assertion and reason both are correct statements and reason is the correct explanation of assertion.

Hence option $\left(C\right)$ is correct.