📘 Detailed Solution
🔵 Step 1: What are s-block elements?
The s-block elements are the elements in which the differentiating electron enters the s-subshell. They mainly include Group 1 (alkali metals) and Group 2 (alkaline earth metals).
🟢 Step 2: Why are they highly reactive?
s-block metals have only one or two electrons in their outermost shell. These electrons can be removed relatively easily because of their comparatively low ionisation enthalpy.
2s¹ → Li⁺ + e⁻
Therefore, they readily lose their valence electrons and form stable positive ions such as Na⁺, K⁺, Mg²⁺ and Ca²⁺.
🟣 Main Reasons for High Reactivity
| Reason | Explanation |
|---|---|
| 1. Few valence electrons | They have only 1 or 2 electrons in the outermost shell. |
| 2. Low ionisation enthalpy | Valence electrons can be removed easily. |
| 3. Large atomic size | The outer electrons are relatively far from the nucleus. |
| 4. Electropositive nature | They readily lose electrons and form cations. |
🎬 Electron-Loss Concept
2,8,1 → Na⁺
2,8
🟠 Examples
Group 1: Li, Na, K, Rb, Cs
Group 2: Be, Mg, Ca, Sr, Ba
Among Group 1 elements, reactivity generally increases down the group because atomic size increases and the outer electron becomes easier to remove.
Reactivity generally increases ↓
✅ Final Answer
s-block elements are called reactive metals because they have only one or two valence electrons and can lose these electrons easily due to their low ionisation enthalpy.
They readily form positive ions and therefore show high chemical reactivity.
📝 20 MCQs with Answers
1. Why are s-block elements highly reactive?
✅ Answer
B) They easily lose their valence electrons
2. Which groups mainly constitute the s-block?
✅ Answer
A) Groups 1 and 2
3. How many valence electrons does a Group 1 element normally have?
✅ Answer
A) 1
4. How many valence electrons does a Group 2 element normally have?
✅ Answer
B) 2
5. s-block elements generally form:
✅ Answer
B) Cations
6. Which of the following is an alkali metal?
✅ Answer
A) Na
7. Which of the following is an alkaline earth metal?
✅ Answer
B) Mg
8. The ionisation enthalpy of alkali metals is generally:
✅ Answer
B) Low
9. Which metal is more reactive?
✅ Answer
D) Rb
10. Reactivity of Group 1 elements generally:
✅ Answer
B) Increases down the group
11. s-block elements are generally:
✅ Answer
A) Electropositive
12. Sodium forms which ion?
✅ Answer
C) Na⁺
13. Calcium forms which ion?
✅ Answer
B) Ca²⁺
14. Which property favours high reactivity of s-block metals?
✅ Answer
B) Low ionisation enthalpy
15. The outer configuration of Group 1 elements is:
✅ Answer
A) ns¹
16. The outer configuration of Group 2 elements is:
✅ Answer
B) ns²
17. Which is the most reactive among the following Group 1 metals?
✅ Answer
D) Cs
18. s-block metals readily react because they tend to achieve:
✅ Answer
A) A stable noble-gas configuration
19. Which of the following belongs to Group 2?
✅ Answer
B) Ca
20. The general electronic configuration of s-block elements is:
✅ Answer
A) ns¹⁻²
Q1. Why are s-block elements called reactive metals?
Solution: They have only one or two valence electrons and low ionisation enthalpy. Hence, they lose their valence electrons easily and form stable cations.
Q2. What is the general electronic configuration of s-block elements?
Solution:
Group 1 elements have ns¹, while Group 2 elements have ns².
Q1. Give three reasons for the high reactivity of s-block elements.
Solution:
- They have only 1 or 2 valence electrons.
- They have relatively low ionisation enthalpy.
- They readily lose electrons and form stable cations.
Q1. Explain the difference between Group 1 and Group 2 metals with respect to reactivity.
| Feature | Group 1 | Group 2 |
|---|---|---|
| Valence electrons | 1 | 2 |
| Configuration | ns¹ | ns² |
| Ion formed | M⁺ | M²⁺ |
| General reactivity | Very high | High, but generally lower than Group 1 |
Q1. Explain why the reactivity of alkali metals increases down the group.
Solution:
- Atomic size increases down the group.
- The outermost electron moves farther from the nucleus.
- Shielding effect also increases.
- Attraction between the nucleus and valence electron decreases.
- Therefore, ionisation enthalpy decreases and electron loss becomes easier.
Reactivity generally increases ↓
Q1. Explain in detail why s-block elements are called reactive metals.
Solution:
s-block elements comprise mainly the elements of Groups 1 and 2. Their general outer electronic configuration is ns¹⁻². Because they possess only one or two valence electrons, they can lose these electrons comparatively easily.
| Factor | Effect |
|---|---|
| Few valence electrons | Easy electron loss |
| Low ionisation enthalpy | Electrons are removed easily |
| Large atomic size | Valence electrons are farther from nucleus |
| Electropositive character | Stable cations are formed |
For example, sodium has the electronic configuration 2,8,1. It readily loses one electron to form Na⁺, which has the stable configuration 2,8.
Thus, s-block metals readily participate in chemical reactions and are therefore called reactive metals.
s-block elements are called reactive metals because their one or two valence electrons are easily lost due to relatively low ionisation enthalpy, resulting in the formation of stable positive ions.
🎯 Quick Revision
General configuration → ns¹⁻²
Valence electrons → 1 or 2
Nature → Electropositive
Ion formation → Cations
Reason for reactivity → Easy loss of valence electrons
Group 1 reactivity → Increases down the group
