📘 Detailed Solution
🔵 Step 1: Electronic Configuration of Group 18
Group 18 elements are called noble gases because their outermost electron shells are completely filled.
ns2 np6
The only exception is Helium (He), whose configuration is:
Thus, helium has a completely filled first shell containing two electrons.
🎬 Electronic Configuration Animation
Watch the outermost shell become completely filled:
Stable Electronic Configuration
🟢 Step 2: Why are they chemically stable?
A completely filled valence shell is a highly stable electronic arrangement. Therefore, noble gases have very little tendency to gain, lose or share electrons.
🟣 Examples
| Element | Electronic Configuration | Valence Electrons |
|---|---|---|
| He | 1s2 | 2 |
| Ne | 1s2 2s2 2p6 | 8 |
| Ar | [Ne] 3s2 3p6 | 8 |
| Kr | [Ar] 3d10 4s2 4p6 | 8 |
✅ Final Answer
Group 18 elements are called noble gases because their outermost electron shells are completely filled. Their general valence-shell configuration is ns²np⁶, except helium, which has 1s². This stable configuration makes them very unreactive or chemically inert.
📝 20 MCQs with Answers
1. Group 18 elements are known as:
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C) Noble gases
2. The general valence-shell configuration of noble gases is:
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B) ns²np⁶
3. Which is the exception to ns²np⁶ configuration?
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C) He
4. Helium has how many valence electrons?
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B) 2
5. Neon has the electronic configuration:
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A) 1s²2s²2p⁶
6. Noble gases have a:
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B) Completely filled outer shell
7. Which noble gas has only one occupied shell?
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A) He
8. Noble gases generally have:
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B) Very low reactivity
9. Which element belongs to Group 18?
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C) Ar
10. The octet is complete when the valence shell contains:
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D) 8 electrons
11. Which noble gas is the lightest?
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A) He
12. Which of the following has a stable octet?
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A) Ne
13. Argon belongs to:
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D) Group 18
14. The low reactivity of noble gases is mainly due to:
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B) Complete valence shell
15. Which one has configuration 1s²2s²2p⁶?
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B) Ne
16. Noble gases generally have:
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A) High ionisation enthalpy
17. Which one is NOT a noble gas?
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C) Cl
18. Group 18 elements are located in the:
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C) Rightmost column
19. Which element has a duplet configuration?
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A) He
20. The term "noble gas" indicates:
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B) Relative chemical inertness
1. Why are noble gases chemically less reactive?
Solution: Noble gases have completely filled valence shells. Therefore, they have little tendency to gain, lose or share electrons.
2. Write the general electronic configuration of Group 18 elements.
Solution: The general configuration is ns²np⁶, except helium, which has 1s².
3. Why is helium placed in Group 18?
Solution: Helium has a completely filled first shell with two electrons and shows the characteristic low reactivity of noble gases.
4. What is meant by a complete valence shell?
Solution: A valence shell is complete when it contains its maximum permitted number of electrons. For most noble gases it contains eight electrons.
5. Write the electronic configuration of neon.
Solution: Ne = 1s² 2s² 2p⁶.
1. Explain why Group 18 elements are called noble gases.
Solution:
- They possess completely filled valence shells.
- Their general configuration is ns²np⁶, except He.
- This stable configuration makes them chemically very less reactive.
2. Compare the electronic configurations of He and Ne.
Solution:
| Element | Configuration | Valence electrons |
|---|---|---|
| He | 1s² | 2 |
| Ne | 1s²2s²2p⁶ | 8 |
3. Explain the relationship between electronic configuration and chemical stability of noble gases.
Solution: Their outermost shells are completely filled. Hence, they have very little tendency to participate in electron transfer or sharing, resulting in high chemical stability and low reactivity.
1. Explain the electronic configuration of Group 18 elements and why it makes them stable.
Solution:
Group 18 elements have completely filled valence shells. Their general configuration is ns²np⁶. Helium is an exception with 1s².
The complete valence shell provides a highly stable arrangement. Therefore, these elements have little tendency to gain, lose or share electrons. Consequently, they are chemically very less reactive.
2. Explain why noble gases generally do not form ions.
Solution:
Noble gases have stable and completely filled valence shells. Removing an electron requires high energy, while adding another electron would require starting a new shell. Hence, formation of simple ions is energetically unfavourable under ordinary conditions.
1. Discuss the electronic configuration and chemical stability of noble gases with examples.
Solution:
Noble gases belong to Group 18 of the periodic table. Their outermost shells are completely filled.
- Helium: 1s²
- Neon: 1s²2s²2p⁶
- Argon: [Ne]3s²3p⁶
- Krypton: [Ar]3d¹⁰4s²4p⁶
Except helium, their general valence-shell configuration is ns²np⁶.
Because their valence shells are complete, they have very little tendency to gain, lose or share electrons. Hence they are highly stable and generally chemically unreactive.
2. Explain why the noble gases have very high ionisation enthalpy and low reactivity.
Solution:
Noble gases have stable electronic configurations with completely filled valence shells. Their electrons are strongly held by the nucleus, so removing an electron requires considerable energy. Therefore, they possess high ionisation enthalpy.
Since gaining or losing electrons is difficult, they generally show very low chemical reactivity.
1. Explain in detail why Group 18 elements are called noble gases. Discuss their electronic configuration, stability and chemical reactivity.
Solution:
Group 18 elements are called noble gases because they possess completely filled valence shells and consequently show very low chemical reactivity.
1️⃣ Electronic Configuration
The general valence-shell configuration is:
The exception is helium:
2️⃣ Complete Valence Shell
The outermost shell of a noble gas is completely filled. This provides a highly stable electronic arrangement.
3️⃣ High Stability
Because of the stable configuration, noble gases do not easily gain, lose or share electrons.
4️⃣ High Ionisation Enthalpy
Their electrons are strongly held by the nucleus. Therefore, a large amount of energy is required to remove an electron.
5️⃣ Low Reactivity
Their tendency to participate in chemical reactions is very low. Hence, they are described as noble or inert gases.
6️⃣ Examples
| Element | Configuration |
|---|---|
| He | 1s² |
| Ne | 1s²2s²2p⁶ |
| Ar | [Ne]3s²3p⁶ |
| Kr | [Ar]3d¹⁰4s²4p⁶ |
🎯 Quick Revision
General Configuration = ns²np⁶
Helium = 1s²
Complete Valence Shell = High Stability
High Stability = Very Low Reactivity
