Impact of penetration effect on Ionization Enthalpy.

1. Impact of penetration effect on Ionization Enthalpy. +

Correct Answer

Greater penetration → stronger nuclear attraction → higher Ionization Enthalpy

Detailed Explanation

The penetration effect refers to the extent to which an electron in an orbital can approach the nucleus and experience its attractive force. For orbitals having the same principal quantum number n, the penetration ability follows:

s > p > d > f

An s-electron has the greatest probability of being found close to the nucleus. Therefore, it experiences a greater effective nuclear attraction.

Greater penetration means the electron is held more strongly by the nucleus, so more energy is required to remove it.

Thus:

Penetration ↑ → Nuclear attraction ↑ → IE ↑

Why is s > p > d > f?

For the same principal shell:

s > p > d > f

The s-orbital has a significant electron probability density very close to the nucleus. p, d and f orbitals have progressively less penetration toward the nucleus. Therefore, the approximate order of shielding experienced by an electron within the same shell is often discussed as:

Effective penetration: s > p > d > f

This is especially important when comparing electrons from different subshells.

Important Examples

Example 1: Beryllium vs Boron Be:

Be: 1s² 2s²

B:

B: 1s² 2s² 2p¹

The electron removed from B is a 2p electron, whereas the electron removed from Be is a 2s electron. Since 2s penetrates more strongly than 2p, the 2s electron in Be is held more strongly. Hence:

IE₁(Be) > IE₁(B)

This explains the famous Be–B ionization enthalpy anomaly.

Example 2: Mg vs Al Mg has:

Mg: [Ne] 3s²

Al has:

Al: [Ne] 3s² 3p¹

The electron removed from Al is a 3p electron, which has less penetration than the 3s electron of Mg. Therefore:

IE₁(Mg) > IE₁(Al)

JEE / NEET Shortcut

Whenever the electron to be removed changes from a more penetrating orbital to a less penetrating orbital, ionization becomes easier.

s-electron removal > p-electron removal

in terms of the energy generally required, when other factors are comparable. Remember the two classic exceptions:

Be > B
Mg > Al

because the electron removed from B/Al is from a less-penetrating p orbital.

30 Related MCQs with Solutions

1. What is the correct order of penetration of orbitals?

A. f > d > p > s

B. s > p > d > f

C. p > s > d > f

D. d > f > s > p

Answer: B. s > p > d > f

2. Greater penetration of an electron generally causes:

A. Weaker nuclear attraction

B. Stronger nuclear attraction

C. Zero attraction

D. Lower nuclear charge

Answer: B

3. Which orbital has the maximum penetration?

A. 2s

B. 2p

C. 3d

D. 4f

Answer: A. 2s

4. Why is IE₁(Be) greater than IE₁(B)?

A. B has fewer protons

B. B loses a 2p electron

C. Be loses a 2p electron

D. Be has a larger radius

Answer: B
B loses a less-penetrating 2p electron.

5. Which electron is easier to remove?

A. 2s

B. 2p

C. 1s

D. All equally

Answer: B. 2p

6. Which orbital penetrates most effectively toward the nucleus?

A. s

B. p

C. d

D. f

Answer: A. s

7. The penetration effect mainly influences:

A. Effective nuclear attraction

B. Atomic mass only

C. Number of neutrons

D. Nuclear decay

Answer: A

8. Which has greater IE₁?

A. Mg

B. Al

C. Both equal

D. Cannot determine

Answer: A. Mg

9. The outer electron of Al is removed from:

A. 3s

B. 3p

C. 2p

D. 3d

Answer: B. 3p

10. The outer electrons of Mg are present in:

A. 2p

B. 3s

C. 3p

D. 2s

Answer: B. 3s

11. Which subshell has the greatest penetration for the same n?

A. ns

B. np

C. nd

D. nf

Answer: A. ns

12. Higher penetration generally means:

A. Electron is more weakly bound

B. Electron is more strongly bound

C. Electron has no energy

D. Electron is absent

Answer: B

13. The Be–B anomaly is related to:

A. 2s vs 2p

B. 1s vs 2s

C. 3s vs 3p

D. 3d vs 4s

Answer: A. 2s vs 2p

14. The Mg–Al anomaly is related to:

A. 2s vs 2p

B. 3s vs 3p

C. 4s vs 4p

D. 3d vs 4p

Answer: B. 3s vs 3p

15. Which is more penetrating?

A. 3s

B. 3p

C. 3d

D. All equal

Answer: A. 3s

16. The order s > p > d > f refers to:

A. Penetration

B. Atomic mass

C. Nuclear spin

D. Number of shells

Answer: A

17. Which electron experiences greater nuclear attraction, other factors being comparable?

A. More penetrating electron

B. Less penetrating electron

C. Both always equal

D. Neither

Answer: A

18. Which orbital is least penetrating among s, p, d and f?

A. s

B. p

C. d

D. f

Answer: D. f

19. If penetration increases, ionization enthalpy generally:

A. Increases

B. Decreases

C. Becomes zero

D. Is unaffected

Answer: A

20. Which electron is held more strongly in a comparable atom?

A. s electron

B. p electron

C. d electron

D. f electron

Answer: A. s electron

21. The penetration effect helps explain which periodic trend anomaly?

A. Be > B in IE

B. Na > Li in IE

C. F > Cl in size

D. K > Na in electronegativity

Answer: A

22. Which electron is closest to the nucleus on average?

A. s electron

B. p electron

C. d electron

D. f electron

Answer: A. s electron

23. Poor penetration generally results in:

A. Stronger nuclear attraction

B. Weaker effective nuclear attraction

C. Larger nuclear charge

D. Complete shielding

Answer: B

24. Which has the highest penetration?

A. 4s

B. 4p

C. 4d

D. 4f

Answer: A. 4s

25. Penetration is related to the probability of finding an electron:

A. Near the nucleus

B. Only outside the atom

C. In the nucleus

D. Nowhere

Answer: A

26. Which statement is correct?

A. Higher penetration means weaker nuclear attraction

B. Higher penetration means stronger nuclear attraction

C. Penetration has no relation to IE

D. f penetrates more than s

Answer: B

27. Which pair correctly represents an ionization-energy anomaly?

A. Be > B

B. Mg > Al

C. Both A and B

D. Neither

Answer: C. Both A and B

28. Why is a p electron generally easier to remove than an s electron of the same shell?

A. p electron has greater penetration

B. p electron has lower penetration

C. p electron has no energy

D. p electron is inside the nucleus

Answer: B

29. Which relationship is correct?

A. Penetration ↑ → IE ↓

B. Penetration ↑ → IE ↑

C. Penetration ↑ → IE = 0

D. No relationship

Answer: B

30. The best summary of the penetration effect is:

A. s electrons penetrate most and are generally held most strongly

B. f electrons penetrate most

C. Penetration decreases nuclear attraction

D. Penetration only affects atomic mass

Answer: A

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