1. How does the atomic radius vary in a group from top to bottom?
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Correct Answer
Atomic radius generally increases from top to bottom in a group.
Explanation
As we move down a group, a new electron shell is added at
each successive element.
For example, in Group 1:
Although the nuclear charge also increases down the group, the increase in
shielding effect and the greater distance of the outermost
electrons from the nucleus dominate.
Therefore, the outermost electron experiences less effective attraction and
the atomic size increases.
Li → Na → K → Rb → Cs
The principal quantum number n increases, so the valence
electrons occupy shells farther from the nucleus.
Li
2 shells
Small radius
Small radius
Na
3 shells
Larger radius
Larger radius
Cs
6 shells
Very large radius
Very large radius
Down a group:
Number of shells ↑ → Shielding ↑ → Atomic radius ↑
Number of shells ↑ → Shielding ↑ → Atomic radius ↑
Why Does Atomic Radius Increase?
1. Addition of new shells:
Each element down a group has an additional principal energy level.
2. Increased shielding effect:
Inner-shell electrons shield the valence electrons from the attraction of the nucleus.
3. Increased distance:
The valence electrons are located farther from the nucleus.
Each element down a group has an additional principal energy level.
2. Increased shielding effect:
Inner-shell electrons shield the valence electrons from the attraction of the nucleus.
3. Increased distance:
The valence electrons are located farther from the nucleus.
Distance from nucleus ↑ + Shielding effect ↑
↓
Atomic radius ↑
↓
Atomic radius ↑
Important Periodic Trend
Remember the two basic trends:
Across a period: Atomic radius ↓
Down a group: Atomic radius ↑
Across a period, electrons are added to the same principal shell while
nuclear charge increases, causing contraction.
Down a group, new shells are added, causing expansion.
Down a group: Atomic radius ↑
JEE Quick Trick
For a simple group comparison:
Top → Bottom = Size increases
Example:
Li < Na < K < Rb < Cs
For halogens:
F < Cl < Br < I
Exception alert: In some comparisons involving transition
or inner-transition elements, simple shell counting is not sufficient.
Effective nuclear charge, poor d/f shielding and contraction effects may
produce anomalies.
30 Related JEE-Level MCQs with Solutions
1. Atomic radius generally increases in a group because:
A. Nuclear charge decreases
B. New electron shells are added
C. Electrons are removed
D. Shielding decreases
Answer: B — A new shell is added down the group.
2. Which has the largest atomic radius?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
3. The shielding effect generally increases:
A. Across a period
B. Down a group
C. Only in noble gases
D. Only in metals
Answer: B. Down a group
4. Which sequence shows increasing atomic size?
A. Cs < Rb < K < Na
B. Na < Li < K < Cs
C. Li < Na < K < Rb < Cs
D. Cs < K < Na < Li
Answer: C
5. The valence electron is farther from the nucleus when moving:
A. Left to right in a period
B. Down a group
C. Right to left only
D. Randomly
Answer: B
6. Which factor strongly contributes to the increase in radius down a group?
A. Decrease in shells
B. Increase in principal quantum number
C. Decrease in shielding
D. Decrease in electron count
Answer: B
7. The number of occupied shells generally:
A. Decreases down a group
B. Increases down a group
C. Remains zero
D. Remains exactly one
Answer: B
8. Which element has the smallest atomic radius?
A. Li
B. Na
C. K
D. Rb
Answer: A. Li
9. Which statement is correct?
A. Atomic radius decreases down every group
B. Atomic radius generally increases down a group
C. Atomic radius never changes
D. Atomic radius depends only on atomic mass
Answer: B
10. The increasing order of halogen atomic size is:
A. I < Br < Cl < F
B. F < Cl < Br < I
C. Cl < F < I < Br
D. Br < I < F < Cl
Answer: B
11. Moving down a group, effective nuclear attraction on the valence electron generally:
A. Becomes dominant enough to decrease radius
B. Is offset by increased distance and shielding
C. Becomes zero
D. Always increases radius directly
Answer: B
12. Which group 17 element is largest?
A. F
B. Cl
C. Br
D. I
Answer: D. I
13. Atomic radius is mainly a measure of:
A. Nuclear mass
B. Size of the electron cloud
C. Number of neutrons only
D. Ionization energy
Answer: B
14. Which quantity increases down a group and contributes to shielding?
A. Number of inner electrons
B. Electronegativity
C. Ionization enthalpy
D. Nuclear stability only
Answer: A
15. Which trend is generally correct?
A. Radius ↑ down group, IE ↓ down group
B. Radius ↓ down group, IE ↑ down group
C. Both always increase
D. Both always decrease
Answer: A
16. Which has the greatest number of occupied shells?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
17. Why does shielding increase down a group?
A. More inner shells are present
B. Nuclear charge disappears
C. Valence electrons disappear
D. Atomic number decreases
Answer: A
18. Which has a larger radius, F or Cl?
A. F
B. Cl
C. Equal
D. Cannot compare
Answer: B. Cl
19. The atomic radius of alkali metals down the group:
A. Increases
B. Decreases
C. Remains constant
D. Becomes zero
Answer: A
20. The dominant reason for increasing size down a group is:
A. Addition of electron shells
B. Decrease in nuclear charge
C. Loss of protons
D. Decrease in atomic number
Answer: A
21. Which is larger?
A. K
B. Na
C. Li
D. All equal
Answer: A. K
22. The outermost electron of Cs is in:
A. 2s
B. 3s
C. 5s
D. 6s
Answer: D. 6s
23. Which has the greatest shielding among Li, Na, K and Cs?
A. Li
B. Na
C. K
D. Cs
Answer: D. Cs
24. As atomic radius increases down a group, the valence electron is generally:
A. More tightly held
B. Less tightly held
C. Absent
D. In the nucleus
Answer: B
25. Which sequence represents increasing size?
A. F < Cl < Br < I
B. I < Br < Cl < F
C. Cl < F < Br < I
D. Br < F < I < Cl
Answer: A
26. Which statement about nuclear charge down a group is true?
A. It decreases
B. It increases
C. It remains zero
D. It is unrelated to atomic number
Answer: B
27. Despite increasing nuclear charge down a group, atomic radius increases because:
A. New shells and shielding dominate
B. Protons are removed
C. Electrons enter the nucleus
D. Atomic number decreases
Answer: A
28. Which periodic trend is opposite to atomic radius down a group?
A. Shielding effect
B. Number of shells
C. Ionization enthalpy
D. Atomic number
Answer: C — Ionization enthalpy generally decreases down a group.
29. If an element is lower in the same group, its valence shell generally has:
A. Lower principal quantum number
B. Higher principal quantum number
C. No electrons
D. The same shell as the element above
Answer: B
30. The best summary of the trend is:
A. Top → bottom: shells ↓, shielding ↓, radius ↓
B. Top → bottom: shells ↑, shielding ↑, radius ↑
C. Top → bottom: shells ↑, shielding ↓, radius ↓
D. Top → bottom: shells unchanged, radius unchanged
Answer: B
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