Ncert Exercises & Solutions

Given below are two statements:

Assertion (A):	Boron has a smaller first ionisation enthalpy than beryllium.
Reason (R):	The penetration of 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.

1.	Both (A) and (R) are True and (R) is the correct explanation of (A).
2.	Both (A) and (R) are True but (R) is not the correct explanation of (A).
3.	(A) is True but (R) is False.
4.	(A) is False but (R) is True.

Hint: Beryllium has a more stable configuration than Boron

Boron has a smaller first ionisation enthalpy than beryllium because the penetration of a 2s electron to the nucleus is more than the 2p electron. Hence, the 2p electron is more shielded by the inner core of the electron than the 2s electron.

The ionization enthalpy values are as follows:
Be = 899 kJ/mol
B = 801 kJ/mol

Hence, Both assertion and reason are true and the reason is the correct explanation of assertion.

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