Shielding effect order: s>p>d>f. Explain its impact on size.

1. Shielding effect order: s > p > d > f. Explain its impact on size. +

Correct Concept

Greater shielding → lower effective nuclear charge → larger atomic size

Explanation

The shielding effect is the reduction in the attractive force of the nucleus on an outer electron due to the presence of inner electrons. The commonly used order of shielding effectiveness is:

s > p > d > f

This means an s-electron shields the nuclear charge more effectively than a p-electron, while an f-electron is the least effective shield. The reason is mainly the difference in penetration of the orbitals.

s Highest shielding
Highest penetration

p High shielding
Less penetration than s

d Poor shielding
Low penetration

f Very poor shielding
Lowest penetration

Impact on Atomic Size

The effective nuclear charge is approximately represented as:

Z_eff = Z − σ

where Z is nuclear charge and σ is the shielding constant. If shielding increases, the outer electron feels less attraction from the nucleus. Therefore:

Shielding ↑ → Z_eff ↓ → Atomic radius ↑

Conversely:

Shielding ↓ → Z_eff ↑ → Atomic radius ↓

Across a Period

Across a period, electrons are added mainly to the same principal shell. Shielding does increase somewhat, but not enough to cancel the increasing nuclear charge. Therefore effective nuclear charge increases:

Z_eff ↑ → attraction ↑ → atomic radius ↓

So atomic size generally decreases from left to right.

Li → Be → B → C → N → O → F : Size ↓

Down a Group

Down a group, new electron shells are added. The number of inner electrons increases substantially, so shielding increases.

Shells ↑ → Shielding ↑ → Atomic radius ↑

Hence atomic size generally increases from top to bottom.

Li < Na < K < Rb < Cs

JEE-Level Key Point

Do not confuse shielding with effective nuclear charge. More shielding means the outer electron experiences a smaller effective nuclear attraction. Therefore:

Shielding ↑ → Z_eff ↓ → Size ↑

For the subshell shielding order:

s > p > d > f

Thus f-electrons shield very poorly. This poor shielding is especially important in explaining phenomena such as lanthanide contraction.

Quick Revision

Shielding ability: s > p > d > f

Shielding ↑ → Z_eff ↓ → Size ↑

Shielding ↓ → Z_eff ↑ → Size ↓

One-line answer: Greater shielding reduces the effective nuclear attraction experienced by outer electrons, thereby increasing the atomic radius.

30 Related JEE-Level MCQs with Solutions

1. The correct order of shielding effectiveness is:

A. s > p > d > f

B. f > d > p > s

C. p > s > d > f

D. d > p > s > f

Answer: A

2. Which orbital provides the maximum shielding?

A. s

B. p

C. d

D. f

Answer: A. s

3. Which orbital has the poorest shielding ability?

A. s

B. p

C. d

D. f

Answer: D. f

4. Greater shielding generally causes atomic radius to:

A. Increase

B. Decrease

C. Become zero

D. Remain unchanged

Answer: A. Increase

5. Greater shielding causes effective nuclear charge to:

A. Increase

B. Decrease

C. Become infinite

D. Become negative

Answer: B. Decrease

6. The approximate expression for effective nuclear charge is:

A. Z_eff = Z + σ

B. Z_eff = Z − σ

C. Z_eff = σ − Z

D. Z_eff = Zσ

Answer: B

7. Which orbital has the greatest penetration power?

A. s

B. p

C. d

D. f

Answer: A. s

8. Penetration and shielding are related because greater penetration generally gives:

A. Better shielding

B. Poorer shielding

C. No shielding

D. No effect

Answer: A. Better shielding

9. Across a period, atomic radius generally:

A. Increases

B. Decreases

C. Remains constant

D. Doubles

Answer: B. Decreases

10. Down a group, atomic radius generally:

A. Decreases

B. Increases

C. Becomes zero

D. Remains constant

Answer: B. Increases

11. The major reason for the increase in atomic size down a group is:

A. Decrease in shells

B. Addition of new electron shells

C. Decrease in nuclear charge

D. Loss of electrons

Answer: B

12. Which electron experiences maximum shielding?

A. An s-electron

B. A p-electron

C. A d-electron

D. An f-electron

Answer: A

13. Poor shielding of 4f electrons is responsible for:

A. Lanthanide contraction

B. Hydrogen bonding

C. Metallic bonding

D. Ionisation only

Answer: A

14. If Z remains constant and shielding increases, the atomic radius generally:

A. Increases

B. Decreases

C. Becomes zero

D. Is unaffected

Answer: A

15. If shielding decreases while nuclear charge remains constant, Z_eff:

A. Increases

B. Decreases

C. Becomes zero

D. Remains unchanged

Answer: A

16. Which has the highest shielding among the following?

A. 4s

B. 4p

C. 4d

D. 4f

Answer: A. 4s

17. Which has the lowest shielding among the following?

A. 3s

B. 3p

C. 3d

D. 4f

Answer: D. 4f

18. Atomic radius depends strongly on:

A. Effective nuclear charge and principal shell

B. Atomic mass alone

C. Neutron number alone

D. Melting point

Answer: A

19. Greater Z_eff generally pulls electrons:

A. Closer to nucleus

B. Farther from nucleus

C. Out of atom

D. Into another atom

Answer: A

20. Which statement is correct?

A. s-electrons shield better than f-electrons

B. f-electrons shield better than s-electrons

C. All orbitals shield equally

D. Shielding is independent of orbital type

Answer: A

21. Moving from Li to F, atomic size generally decreases because:

A. Nuclear charge increases significantly

B. Shell number increases

C. Shielding increases enormously

D. Electrons are removed

Answer: A

22. Moving down Group 1, atomic radius increases mainly because:

A. More electron shells are added

B. Nuclear charge decreases

C. Electrons disappear

D. Nuclear radius becomes zero

Answer: A

23. Which subshell has maximum penetration?

A. ns

B. np

C. nd

D. nf

Answer: A. ns

24. Which is the correct relationship?

A. Shielding ↑ → Z_eff ↑

B. Shielding ↑ → Z_eff ↓

C. Shielding ↓ → Z_eff ↓

D. No relationship

Answer: B

25. A decrease in atomic radius generally indicates:

A. Stronger effective nuclear attraction

B. Weaker nuclear attraction

C. More shells only

D. Complete shielding

Answer: A

26. The poorest shielding by f-electrons is associated with their:

A. Poor penetration

B. Highest penetration

C. Absence from atoms

D. Zero charge

Answer: A

27. Which factor tends to increase atomic size?

A. Increased shielding

B. Increased Z_eff

C. Increased nuclear attraction

D. Increased ionization energy

Answer: A

28. Which factor tends to decrease atomic size?

A. Increased effective nuclear charge

B. Increased shielding

C. Addition of shells

D. Increased electron-electron repulsion only

Answer: A

29. The correct sequence for the effect of shielding on size is:

A. Shielding ↑ → Z_eff ↓ → size ↑

B. Shielding ↑ → Z_eff ↑ → size ↓

C. Shielding ↓ → Z_eff ↓ → size ↑

D. Shielding has no effect

Answer: A

30. Which is the best one-line conclusion?

A. More shielding means smaller atomic size

B. More shielding means larger atomic size

C. Shielding does not affect size

D. More shielding always increases nuclear charge

Answer: B. More shielding means larger atomic size

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