Which of the following has the highest 1st I.E.: B,C,N,O?

1. Which of the following has the highest 1st I.E.: B, C, N, O? +

Correct Answer

Nitrogen (N)

Detailed Solution

Across the second period, first ionization enthalpy generally increases from left to right because effective nuclear charge increases and atomic radius decreases.

B → C → N → O

However, there is an important exception between N and O. Electronic configurations:

B = 1s² 2s² 2p¹
C = 1s² 2s² 2p²
N = 1s² 2s² 2p³
O = 1s² 2s² 2p⁴

Nitrogen has a particularly stable half-filled 2p³ configuration. In nitrogen, the three 2p electrons occupy the three p orbitals singly:

2p³ → ↑   ↑   ↑

This half-filled arrangement has extra stability due to symmetrical distribution and exchange energy. Oxygen has:

2p⁴ → ↑↓   ↑   ↑

One of the 2p orbitals contains a paired set of electrons. The electron-electron repulsion within this paired orbital makes removal of one electron from oxygen relatively easier. Therefore:

IE₁(B) < IE₁(C) < IE₁(O) < IE₁(N)

Hence, among B, C, N and O, the highest first ionization enthalpy is Nitrogen.

JEE / NEET Shortcut

Remember the important second-period exceptions:

Be > B

because Be has a stable filled 2s² subshell, and

N > O

because N has the stable half-filled 2p³ configuration. So for this question:

B < C < O < N

Therefore:

N is the highest.

30 Related MCQs with Solutions

1. Which has the highest first ionization enthalpy among B, C, N and O?

A. B

B. C

C. N

D. O

Answer: C. N
N has the stable half-filled 2p³ configuration.

2. The electronic configuration of nitrogen is:

A. 1s²2s²2p¹

B. 1s²2s²2p²

C. 1s²2s²2p³

D. 1s²2s²2p⁴

Answer: C. 1s²2s²2p³

3. Why is IE₁ of N higher than O?

A. N has larger size

B. N has a half-filled p subshell

C. O has lower nuclear charge

D. N has fewer shells

Answer: B

4. The configuration 2p³ represents:

A. B

B. C

C. N

D. O

Answer: C. N

5. Which configuration is half-filled?

A. p¹

B. p²

C. p³

D. p⁴

Answer: C. p³

6. Which subshell is half-filled in nitrogen?

A. 1s

B. 2s

C. 2p

D. 3s

Answer: C. 2p

7. The IE₁ order of B, C, N and O is:

A. B < C < O < N

B. B < O < C < N

C. N < O < C < B

D. O < N < C < B

Answer: A

8. Oxygen has configuration:

A. 2p²

B. 2p³

C. 2p⁴

D. 2p⁵

Answer: C. 2p⁴

9. The paired electrons in oxygen cause:

A. Increased repulsion

B. Half-filled stability

C. Larger nuclear charge

D. Zero shielding

Answer: A. Increased electron-electron repulsion

10. Which element has the highest effective nuclear charge among B, C, N and O?

A. B

B. C

C. N

D. O

Answer: D. O
But IE₁ is still higher for N because of the half-filled p³ stability.

11. Which is an important ionization-energy exception?

A. Li > Be

B. Be > B

C. C > N

D. N < O

Answer: B. Be > B

12. Which element has a stable half-filled p subshell?

A. C

B. N

C. O

D. F

Answer: B. N

13. The term "exchange energy" is associated with:

A. Half-filled and fully-filled subshell stability

B. Atomic mass

C. Nuclear decay

D. Melting point

Answer: A

14. Which has greater IE₁: N or O?

A. N

B. O

C. Equal

D. Cannot determine

Answer: A. N

15. Which has the configuration 1s²2s²2p¹?

A. B

B. C

C. N

D. O

Answer: A. B

16. Across a period, ionization enthalpy generally:

A. Decreases

B. Increases

C. Remains constant

D. Becomes zero

Answer: B. Increases

17. Which factor generally increases IE across a period?

A. Increasing atomic radius

B. Increasing effective nuclear charge

C. Increasing shielding drastically

D. Increasing shells

Answer: B

18. Which has the smallest atomic radius among B, C, N and O?

A. B

B. C

C. N

D. O

Answer: D. O

19. The relatively lower IE of O compared with N is due mainly to:

A. Electron pairing in 2p

B. Larger number of shells

C. Lower atomic number

D. Absence of p electrons

Answer: A

20. Which has three unpaired electrons in its 2p subshell?

A. B

B. C

C. N

D. O

Answer: C. N

21. Which has one paired orbital in its 2p subshell?

A. B

B. C

C. N

D. O

Answer: D. O

22. The stability of nitrogen is associated with:

A. p¹

B. p²

C. p³

D. p⁴

Answer: C. p³

23. Which principle explains the distribution of electrons among degenerate p orbitals?

A. Hund's rule

B. Aufbau principle only

C. Heisenberg principle

D. Octet rule

Answer: A. Hund's rule

24. Which arrangement is most stable?

A. p¹

B. p²

C. p³

D. p⁴

Answer: C. p³

25. Which element has the highest IE₁ among C, N and O?

A. C

B. N

C. O

D. All equal

Answer: B. N

26. Which is easier to ionize, N or O?

A. N

B. O

C. Both equal

D. Cannot say

Answer: B. O

27. Which statement is correct?

A. O has a half-filled 2p subshell

B. N has a half-filled 2p subshell

C. C has a half-filled 2p subshell

D. B has a half-filled 2p subshell

Answer: B

28. The electronic configuration of carbon is:

A. 2p¹

B. 2p²

C. 2p³

D. 2p⁴

Answer: B. 2p²

29. The correct increasing order of first ionization enthalpy is:

A. B < C < O < N

B. N < O < C < B

C. O < N < C < B

D. B < O < C < N

Answer: A

30. The main reason for the N > O ionization-energy anomaly is:

A. Half-filled 2p³ stability of N

B. Filled 2p⁶ configuration of O

C. Larger size of N

D. Lower nuclear charge of O

Answer: A

Advertisement