📘 Answer
Noble gases have large positive electron gain enthalpy (EGE) because they possess a completely filled and highly stable valence shell. Their electronic configurations are ns²np⁶, except helium, which has 1s².
Ne → 2s² 2p⁶
Ar → 3s² 3p⁶
If an additional electron is added to a noble gas atom, it cannot enter the already completely filled valence shell. It must enter a new, higher-energy shell.
⚛️ Why is the value positive?
- Noble gases already have a stable complete valence shell.
- Adding another electron disturbs this stable configuration.
- The incoming electron must occupy a new, higher-energy orbital.
- Energy must therefore be supplied to add the electron.
- Hence, their electron gain enthalpy is large and positive.
🔹 Example: Neon
Neon has the electronic configuration:
Its second shell is completely filled. An additional electron would have to enter the third shell, which is higher in energy. Therefore, energy is required for electron addition.
✅ Final Answer
Noble gases have large positive electron gain enthalpy because they possess a completely filled and stable valence shell. An additional electron has to enter a new, higher-energy shell and is also repelled by the existing electrons. Therefore, energy must be supplied, making their EGE large and positive.
20 MCQs with Answers
1. Noble gases have large positive EGE because they have:
✅ Answer
B) Completely filled valence shell
2. The general valence-shell configuration of noble gases is:
✅ Answer
B) ns²np⁶
3. The exception to ns²np⁶ among noble gases is:
✅ Answer
C) He
4. Helium has the electronic configuration:
✅ Answer
B) 1s²
5. When a noble gas gains an electron, the electron enters:
✅ Answer
C) A new higher-energy shell
6. Positive EGE means:
✅ Answer
B) Energy is absorbed
7. Noble gases are chemically stable mainly because of:
✅ Answer
B
8. Which element has a completely filled second shell?
✅ Answer
C) Ne
9. Which noble gas has configuration 1s²?
✅ Answer
A) He
10. Adding an electron to a noble gas generally:
✅ Answer
B) Requires energy
11. Which group contains noble gases?
✅ Answer
D) Group 18
12. Which has a more stable valence-shell configuration?
✅ Answer
C) Ne
13. Noble gases have very low tendency to:
✅ Answer
B
14. Which of the following has the configuration ns²np⁶?
✅ Answer
B
15. The incoming electron in a noble gas experiences:
✅ Answer
B
16. Which has a completely filled octet?
✅ Answer
A) Ne
17. Noble gases have high resistance to electron addition because:
✅ Answer
A
18. Which statement is correct?
✅ Answer
B
19. The stable configuration of neon is:
✅ Answer
C) 1s²2s²2p⁶
20. The main reason for positive EGE of noble gases is:
✅ Answer
B
Q. Why do noble gases have large positive electron gain enthalpy?
Solution: Noble gases have completely filled and stable valence shells. An additional electron must enter a new, higher-energy shell and is strongly repelled by the existing electrons. Therefore, energy is required and their EGE is large and positive.
Q. Explain the reason for the positive electron gain enthalpy of noble gases.
Solution:
- Noble gases have completely filled valence shells.
- They have highly stable electronic configurations.
- An additional electron has to enter a new and higher-energy shell.
- Therefore, energy must be supplied for electron addition.
Q. Explain why noble gases have positive EGE, whereas halogens generally have negative EGE.
Solution:
| Noble Gases | Halogens |
|---|---|
| Valence shell completely filled | One electron short of stable configuration |
| Additional electron enters a new shell | Additional electron completes the valence shell |
| Energy is required | Energy is generally released |
| Positive EGE | Negative EGE |
Q. Discuss the electronic configuration and electron gain enthalpy of noble gases.
Solution:
Noble gases are placed in Group 18 of the periodic table. Except helium, their general valence-shell configuration is ns²np⁶.
Ne = 1s²2s²2p⁶
Ar = [Ne]3s²3p⁶
These configurations represent completely filled and highly stable valence shells. Therefore, noble gases have very little tendency to accept an additional electron.
If an electron is added, it has to enter a new, higher-energy shell. The incoming electron also experiences repulsion from the electrons already present. Hence, energy is absorbed.
Q. Explain in detail why noble gases have large positive electron gain enthalpy.
Detailed Solution
Noble gases are chemically stable elements belonging to Group 18. Their stability arises from their completely filled valence shells.
1. Stable Electronic Configuration
Except helium, noble gases have the general configuration:
Helium has the stable configuration:
2. Addition of an Electron
When an additional electron is supplied to a noble gas atom, the existing valence shell is already completely filled. Therefore, the new electron cannot be accommodated in the same shell without violating the available orbital capacity.
It must occupy an orbital in the next higher principal energy level. This requires energy.
3. Electron-Electron Repulsion
The incoming electron is also repelled by the electrons already present in the atom. Thus, additional energy is required to force the electron into the atom.
↓
Very stable configuration
↓
Incoming electron enters higher-energy shell
↓
Energy absorbed
↓
Large positive EGE
4. Comparison with Halogens
Halogens have seven valence electrons. They can achieve a stable noble-gas configuration by accepting one electron. Therefore, their first electron gain enthalpy is generally negative.
Noble gases already possess the stable configuration, so accepting another electron is energetically unfavourable.
Noble gases have large positive EGE because their valence shells are already completely filled and highly stable. An additional electron must enter a higher-energy shell and experiences electron-electron repulsion. Hence, energy must be supplied, resulting in a large positive electron gain enthalpy.
🎯 Quick Revision
Configuration: ns²np⁶ (He = 1s²)
Valence Shell: Completely filled
Electron Addition: Unfavourable
Incoming Electron: Enters higher-energy shell
Result: Energy absorbed
EGE: Large positive
