📘 Screening Effect / Shielding Effect
The screening effect is the reduction in the attractive force of the nucleus on the outermost electron due to the presence of inner-shell electrons.
The inner electrons act as a shield between the positively charged nucleus and the valence electrons.
Inner electrons shield the valence electron from the nucleus.
🔹 Effect on Ionization Enthalpy
When the screening effect increases, the outermost electron experiences less effective nuclear attraction. Therefore, the electron is held less strongly and can be removed more easily.
Thus, an increase in screening effect generally results in a decrease in ionization enthalpy.
📈 Trend Down a Group
As we move down a group, new electron shells are added. The number of inner-shell electrons increases, so the shielding effect also increases.
| Down the Group | Effect |
|---|---|
| Number of shells ↑ | Shielding effect ↑ |
| Atomic size ↑ | Distance of valence electron from nucleus ↑ |
| Nuclear attraction on valence electron ↓ | Ionization enthalpy ↓ |
⚛️ Example: Alkali Metals
Consider the Group 1 elements:
Down the group, more inner shells are added. These inner electrons shield the valence electron from the nucleus. Hence, the outermost electron is easier to remove.
⚠️ Important Point
The shielding effect is one of the major factors affecting ionization enthalpy. However, ionization enthalpy also depends on atomic size, effective nuclear charge, and electronic configuration.
✅ Final Answer
The screening or shielding effect reduces the effective attraction of the nucleus on the outermost electron. As shielding increases, the valence electron is held less strongly and can be removed more easily. Therefore, ionization enthalpy decreases with an increase in shielding effect.
20 MCQs with Answers
1. Screening effect is caused mainly by:
✅ Answer
B) Inner-shell electrons
2. Screening effect reduces:
✅ Answer
C
3. When shielding effect increases, ionization enthalpy generally:
✅ Answer
B) Decreases
4. Inner electrons act as a:
✅ Answer
B) Shield
5. Shielding effect generally increases:
✅ Answer
B) Down a group
6. Down a group, the number of electron shells:
✅ Answer
B) Increases
7. Greater shielding makes the valence electron:
✅ Answer
B
8. Greater shielding generally causes atomic radius to:
✅ Answer
A) Increase
9. Ionization enthalpy is the energy required to remove:
✅ Answer
C) Electron
10. Which group shows a clear increase in shielding down the group?
✅ Answer
A) Group 1
11. Effective nuclear charge experienced by a valence electron is reduced by:
✅ Answer
A) Shielding
12. Which is easier when shielding is greater?
✅ Answer
A
13. The outermost electron is also called:
✅ Answer
B) Valence electron
14. Which factor generally decreases ionization enthalpy down a group?
✅ Answer
A
15. Shielding is also called:
✅ Answer
A) Screening effect
16. If shielding increases, the attraction between nucleus and valence electron:
✅ Answer
B) Decreases
17. Which has generally greater shielding?
✅ Answer
C) K
18. Increased shielding is associated with:
✅ Answer
A
19. Which statement is correct?
✅ Answer
B
20. Correct relationship is:
✅ Answer
B) Shielding ↑ → IE ↓
Q. How does shielding effect affect ionization enthalpy?
The inner-shell electrons shield the valence electron from the nucleus. Therefore, the effective nuclear attraction decreases and the valence electron becomes easier to remove.
Q. Explain the role of shielding effect in determining ionization enthalpy.
- Inner-shell electrons shield the valence electrons from the nucleus.
- Greater shielding reduces effective nuclear attraction.
- The valence electron is held less strongly.
- Hence, less energy is required for its removal.
Q. Explain the relationship between shielding effect and ionization enthalpy down a group.
As we move down a group, new electron shells are added. The number of inner-shell electrons increases and consequently the shielding effect increases.
- Number of shells ↑
- Shielding effect ↑
- Effective nuclear attraction on valence electron ↓
- Atomic size ↑
- Ionization enthalpy ↓
Q. What is shielding effect? Explain its effect on ionization enthalpy with a suitable example.
The shielding effect is the reduction in the effective nuclear attraction experienced by an outer electron due to the presence of inner-shell electrons.
For example, consider Li and Na. Sodium has an additional electron shell compared with lithium. The inner electrons in sodium shield its valence electron from the nucleus more effectively.
Na: 2,8,1
Therefore, the outer electron of Na is farther from the nucleus and is more strongly shielded. It is easier to remove than the valence electron of Li. Hence, sodium has lower first ionization enthalpy than lithium.
Q. Define screening effect and explain in detail how it affects ionization enthalpy.
Detailed Solution
The screening effect, also called the shielding effect, is the reduction in the attractive force of the nucleus on the outermost electron due to the repulsion and shielding provided by inner-shell electrons.
Effect on Ionization Enthalpy
- Inner electrons lie between the nucleus and valence electrons.
- They partially shield the valence electrons from nuclear attraction.
- Thus, the effective nuclear attraction experienced by the valence electron decreases.
- The valence electron becomes farther from and less strongly bound to the nucleus.
- Consequently, less energy is required to remove the electron.
- Therefore, ionization enthalpy decreases as the shielding effect increases.
↓
Effective Nuclear Attraction ↓
↓
Valence Electron Held Less Strongly
↓
Ionization Enthalpy ↓
Example
Down Group 1, the shielding effect increases:
As a result, the outermost electron becomes progressively easier to remove, so the first ionization enthalpy generally decreases down the group.
An increase in screening effect reduces the effective nuclear attraction on the valence electron and therefore lowers the ionization enthalpy.
Shielding ↑ → Effective Nuclear Charge ↓ → IE ↓
🎯 Quick Revision
Down a group: Shielding ↑
Effective nuclear attraction: ↓
Atomic size: ↑
Ionization enthalpy: ↓
Shielding ↑ → IE ↓
