Arrange the following in decreasing order of atomic size: Li,Na,K,Rb,Cs.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Solution

🔵 Periodic Trend

All the given elements belong to Group 1 (Alkali Metals) of the periodic table.

As we move down a group, a new electron shell is added at each step. Therefore, the atomic radius generally increases because the outermost electron is farther from the nucleus and shielding effect also increases.

Atomic size increases ↓ down the group

📊 Given Elements

Element	Period	Shells	Relative Atomic Size
Li	2	2	Smallest
Na	3	3	↑
K	4	4	↑
Rb	5	5	↑
Cs	6	6	Largest

⬇️ Decreasing Order

Since atomic size increases from Li to Cs, the decreasing order is the reverse:

Cs > Rb > K > Na > Li

💡 Reason

• All elements belong to Group 1.
• Down the group, the number of electron shells increases.
• Shielding effect increases.
• The outermost electron becomes farther from the nucleus.
• Hence atomic radius increases from Li to Cs.

Li < Na < K < Rb < Cs
(Increasing atomic size)

✅ Final Answer

Decreasing order of atomic size:

Cs > Rb > K > Na > Li

📝 20 MCQs with Answers

1. Which is the largest atom among Li, Na, K, Rb and Cs?

A) Li

B) Na

C) K

D) Cs

✅ Answer

D) Cs

---

2. Which is the smallest atom among the given elements?

A) Li

B) Na

C) Rb

D) Cs

✅ Answer

A) Li

---

3. All the given elements belong to:

A) Group 1

B) Group 2

C) Group 17

D) Group 18

✅ Answer

A) Group 1

---

4. Atomic size generally ______ down a group.

A) Decreases

B) Increases

C) Remains constant

D) Becomes zero

✅ Answer

B) Increases

---

5. The correct decreasing order is:

A) Li > Na > K > Rb > Cs

B) Cs > Rb > K > Na > Li

C) K > Li > Na > Cs > Rb

D) Na > Li > Cs > K > Rb

✅ Answer

B) Cs > Rb > K > Na > Li

---

6. Which factor increases down Group 1?

A) Number of shells

B) Nuclear size only

C) Electronegativity

D) Ionisation energy

✅ Answer

A) Number of shells

---

7. Shielding effect down a group generally:

A) Decreases

B) Increases

C) Becomes zero

D) Remains exactly constant

✅ Answer

B) Increases

---

8. Which element has the highest principal quantum number among the given elements?

A) Li

B) Na

C) K

D) Cs

✅ Answer

D) Cs

---

9. Li belongs to which period?

A) 1

B) 2

C) 3

D) 4

✅ Answer

B) 2

---

10. Cs belongs to which period?

A) 4

B) 5

C) 6

D) 7

✅ Answer

C) 6

---

11. Which element has three electron shells?

A) Li

B) Na

C) K

D) Rb

✅ Answer

B) Na

---

12. Which has four electron shells?

A) Li

B) Na

C) K

D) Cs

✅ Answer

C) K

---

13. Atomic radius increases down Group 1 mainly because:

A) New shells are added

B) Electrons are removed

C) Nuclear charge decreases

D) Protons disappear

✅ Answer

A) New shells are added

---

14. Which has the greatest shielding effect?

A) Li

B) Na

C) Rb

D) Cs

✅ Answer

D) Cs

---

15. The outermost electron of Cs is located in:

A) 2nd shell

B) 3rd shell

C) 5th shell

D) 6th shell

✅ Answer

D) 6th shell

---

16. Which sequence represents increasing atomic size?

A) Cs < Rb < K < Na < Li

B) Li < Na < K < Rb < Cs

C) K < Na < Li < Cs < Rb

D) Rb < Cs < Li < Na < K

✅ Answer

B) Li < Na < K < Rb < Cs

---

17. Which pair has the larger atomic radius?

A) Li > Na

B) Na > K

C) K > Na

D) Cs < Rb

✅ Answer

C) K > Na

---

18. Rb is ______ in size compared with K.

A) Smaller

B) Larger

C) Equal

D) Unrelated

✅ Answer

B) Larger

---

19. The correct trend in atomic radius for Group 1 is:

A) Li > Na > K > Rb > Cs

B) Li < Na < K < Rb < Cs

C) Li = Na = K = Rb = Cs

D) Random

✅ Answer

B) Li < Na < K < Rb < Cs

---

20. Which statement is correct?

A) Atomic size decreases down Group 1

B) Atomic size increases down Group 1

C) Atomic size is independent of shells

D) Cs is the smallest among them

✅ Answer

B) Atomic size increases down Group 1

Q. Arrange Li, Na, K, Rb and Cs in decreasing order of atomic size. Give reason.

Solution:

All these elements belong to Group 1. Atomic size increases down the group because the number of electron shells and shielding effect increase.

Cs > Rb > K > Na > Li

Q. Explain the trend in atomic size of Li, Na, K, Rb and Cs.

Solution:

1. Li, Na, K, Rb and Cs belong to Group 1.
2. Moving down the group, a new electron shell is added.
3. Shielding effect increases and the outermost electron is farther from the nucleus.
4. Therefore, atomic size increases down the group.

Li < Na < K < Rb < Cs
Increasing atomic size

Q. Arrange Li, Na, K, Rb and Cs in decreasing order of atomic size and explain the trend.

Element	Period	Atomic Size Trend
Li	2	Smallest
Na	3	↑
K	4	↑
Rb	5	↑
Cs	6	Largest

The atomic size increases down the group because additional electron shells are added and shielding effect increases.

Decreasing order: Cs > Rb > K > Na > Li

Q. Discuss the periodic trend of atomic size down Group 1 using Li, Na, K, Rb and Cs.

Solution:

The alkali metals Li, Na, K, Rb and Cs are placed in Group 1. On moving from Li to Cs, the principal quantum number of the valence shell increases. Thus, each successive element contains one additional electron shell.

• Number of shells increases.
• Shielding effect increases.
• Distance of the valence electron from the nucleus increases.
• Effective attraction on the outermost electron decreases.
• Therefore, atomic radius increases down the group.

Increasing size:
Li < Na < K < Rb < Cs

Decreasing size:

Cs > Rb > K > Na > Li

Q. Arrange Li, Na, K, Rb and Cs in decreasing order of atomic size. Explain the periodic trend in detail.

Detailed Solution

Li, Na, K, Rb and Cs are alkali metals belonging to Group 1 of the periodic table. Their valence-shell configuration is generally ns¹.

Element	Electronic Configuration (Shell-wise)	Period
Li	2, 1	2
Na	2, 8, 1	3
K	2, 8, 8, 1	4
Rb	2, 8, 18, 8, 1	5
Cs	2, 8, 18, 18, 8, 1	6

Why does atomic size increase?

1. A new principal shell is added as we move down the group.
2. The valence electron is placed farther from the nucleus.
3. The number of inner-shell electrons increases.
4. Therefore, shielding effect increases.
5. The effective attraction of the nucleus on the valence electron decreases.
6. Hence, atomic radius increases from Li to Cs.

Down Group 1:
Number of shells ↑ → Shielding effect ↑ → Atomic radius ↑

Final Arrangement

Decreasing order of atomic size:

Cs > Rb > K > Na > Li

Increasing order:
Li < Na < K < Rb < Cs

🎯 Quick Revision

Group: 1
Elements: Li, Na, K, Rb, Cs
Trend down group: Atomic size ↑
Reason: New shells + increased shielding effect

Cs > Rb > K > Na > Li