The correct order of first ionization enthalpy values of the following elements is:

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Solution

Q.
The correct order of first ionization enthalpy values of the following elements is:

(A) O

(B) N

(C) Be

(D) F

(E) B

Choose the correct answer from the options given below :

1 E < C < A < B < D

2 B < D < C < E < A

3 C < E < A < B < D

4 A < B < D < C < E

Ans.
(1)

Ionization enthalpy increases from left to right across a period. Thus in second period element expected order of ionization enthalpy is:

$Li (2 s^{1}) < Be (2 s^{2}) < B (2 s^{2} 2 p^{1}) < C (2 s^{2} 2 p^{2}) < N (2 s^{2} 2 p^{3}) < O (2 s^{2} 2 p^{4}) < F (2 s^{2} 2 p^{5}) < Ne (2 s^{2} 2 p^{6})$

But removal of electron from stable fully filled subshell of Be and half flled subshell of N is difficult. Thus Be and N have ionization energy more than expected value. Hence actual order of ionization energy is:

$Li (2 s^{1}) < B (2 s^{2} 2 p^{1}) < Be (2 s^{2}) < C (2 s^{2} 2 p^{2}) < O (2 s^{2} 2 p^{4}) < N (2 s^{2} 2 p^{3}) < F (2 s^{2} 2 p^{5}) < Ne (2 s^{2} 2 p^{6})$

(E) (C) (A) (B) (D)

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