Why is the atomic radius of Gallium smaller than Aluminum? (JEE Level)

1. Why is the atomic radius of Gallium smaller than Aluminum? (JEE Level) +

Correct Answer

Gallium has a slightly smaller atomic radius than Aluminium due to poor shielding by the intervening 3d electrons.

JEE-Level Explanation

Normally, atomic radius is expected to increase down a group because a new electron shell is added. However, for Group 13:

Al  →  Ga

the expected increase in size does not occur.

Aluminium (Al) Z = 13
[Ne] 3s² 3p¹

Gallium (Ga) Z = 31
[Ar] 3d¹⁰ 4s² 4p¹

In Ga, the additional electrons occupy the 3d subshell. The 3d electrons shield the nuclear charge poorly. Therefore, the outer 4p electron experiences a relatively high effective nuclear charge.

Poor 3d shielding → Z_eff ↑ → Attraction of nucleus ↑ → Atomic radius ↓

Thus, although Ga has an additional shell compared with Al, the increase in nuclear charge together with poor shielding by 3d electrons causes the Ga atom to be contracted. Hence:

r(Ga) < r(Al)

Why is 3d Shielding Poor?

The penetration and shielding ability of orbitals generally follow:

s > p > d > f

Therefore, d-electrons are relatively ineffective at shielding the outer electrons from the nuclear charge. In Ga, the filled 3d¹⁰ subshell lies between the nucleus and the outer 4s/4p electrons. Its poor shielding makes the effective nuclear charge on the outer electrons relatively high.

Important JEE Concept

This is an important exception to the simple rule that atomic size increases down a group. The effect is also related to the phenomenon often described as d-block contraction.

Al → Ga : Atomic radius decreases slightly

Do not confuse this with lanthanoid contraction. The Al–Ga anomaly is mainly associated with the poor shielding effect of the filled 3d¹⁰ subshell.

30 Related JEE-Level MCQs with Solutions

1. The atomic radius of Ga is smaller than that of Al mainly because of:

A. Smaller nuclear charge of Ga

B. Poor shielding by 3d electrons

C. Absence of d electrons in Ga

D. Larger size of 4p orbital

Answer: B — 3d electrons shield poorly.

2. The electronic configuration of Ga is:

A. [Ne] 3s²3p¹

B. [Ar] 3d¹⁰4s²4p¹

C. [Ar] 3d⁵4s²4p²

D. [Kr] 4d¹⁰5s²5p¹

Answer: B

3. Which subshell is responsible for the unusual Al–Ga size relationship?

A. 2p

B. 3p

C. 3d

D. 4d

Answer: C. 3d

4. The shielding ability of a d-electron is generally:

A. Very high

B. Poor

C. Equal to s-electron

D. Zero

Answer: B. Poor

5. The effective nuclear charge experienced by Ga valence electrons is relatively:

A. Lower than expected

B. Higher due to poor 3d shielding

C. Zero

D. Equal to that of Al

Answer: B

6. Which statement is correct?

A. Ga is larger than Al by a large amount

B. Ga and Al have identical radii

C. Ga is slightly smaller than Al

D. Al is smaller because it has fewer shells

Answer: C

7. Poor shielding causes effective nuclear charge to:

A. Decrease

B. Increase

C. Become zero

D. Remain unchanged

Answer: B

8. Which orbital has the greatest penetration?

A. s

B. p

C. d

D. f

Answer: A. s

9. Which order represents decreasing shielding effectiveness?

A. s > p > d > f

B. f > d > p > s

C. p > s > d > f

D. d > f > p > s

Answer: A

10. The atomic numbers of Al and Ga are:

A. 12 and 30

B. 13 and 31

C. 14 and 32

D. 13 and 30

Answer: B

11. Ga belongs to which period?

A. 3

B. 4

C. 5

D. 6

Answer: B. Period 4

12. Al belongs to which period?

A. 2

B. 3

C. 4

D. 5

Answer: B. Period 3

13. Al and Ga belong to:

A. Group 1

B. Group 2

C. Group 13

D. Group 17

Answer: C. Group 13

14. Which phenomenon explains contraction after filling of d orbitals?

A. Inert pair effect only

B. d-block contraction

C. Hydrogen bonding

D. Metallic bonding

Answer: B. d-block contraction

15. In Ga, the 3d subshell contains:

A. 2 electrons

B. 5 electrons

C. 10 electrons

D. 14 electrons

Answer: C. 10 electrons

16. The valence-shell configuration of Ga is:

A. 4s²4p¹

B. 3s²3p¹

C. 3d¹⁰4p²

D. 5s²5p¹

Answer: A. 4s²4p¹

17. Which effect increases nuclear attraction on Ga's outer electrons?

A. Excellent shielding

B. Poor 3d shielding

C. Electron-electron repulsion only

D. Decreased nuclear charge

Answer: B

18. The unusual size relationship is an example of:

A. Periodic anomaly

B. Isotopic effect

C. Hydrogen bonding

D. Nuclear fission

Answer: A

19. Which statement about 3d electrons is correct?

A. They shield outer electrons very efficiently

B. They shield outer electrons relatively poorly

C. They do not exist in Ga

D. They are valence p electrons

Answer: B

20. Atomic radius generally decreases when effective nuclear charge:

A. Increases

B. Decreases

C. Becomes zero

D. Is unchanged

Answer: A

21. Which has greater effective nuclear charge, approximately?

A. Al valence electron

B. Ga valence electron due to poor d shielding

C. Both exactly equal

D. Neither

Answer: B

22. Which pair demonstrates d-block contraction?

A. Li–Na

B. Al–Ga

C. C–Si

D. F–Cl

Answer: B. Al–Ga

23. The presence of 3d electrons in Ga results in:

A. Strong shielding

B. Poor shielding

C. Removal of nuclear charge

D. Removal of 4p electrons

Answer: B

24. Which factor opposes the expected increase in radius from Al to Ga?

A. Increased principal quantum number

B. Increased nuclear charge and poor 3d shielding

C. Decreased number of protons

D. Absence of inner electrons

Answer: B

25. Which statement is most accurate for Ga?

A. 3d electrons shield completely

B. 3d electrons provide poor shielding

C. Ga has no d electrons

D. Ga has only one shell

Answer: B

26. The anomaly in Al and Ga radii is primarily associated with:

A. Poor shielding by d electrons

B. Strong shielding by s electrons

C. Absence of nuclear charge

D. Complete ionisation

Answer: A

27. Which electronic configuration belongs to Al?

A. [Ne]3s²3p¹

B. [Ar]3d¹⁰4s²4p¹

C. [He]2s²2p¹

D. [Ne]3d¹4s²

Answer: A

28. The key difference between Al and Ga is the presence of:

A. 2p electrons in Ga

B. 3d electrons in Ga

C. 4f electrons in Al

D. 5d electrons in Al

Answer: B

29. If shielding is poor, the outer electron is attracted to the nucleus:

A. Less strongly

B. More strongly

C. Not at all

D. Randomly

Answer: B. More strongly

30. The correct conclusion is:

A. r(Ga) > r(Al) because Ga has an extra shell

B. r(Ga) < r(Al) because 3d electrons shield poorly

C. r(Ga) = r(Al) exactly

D. Ga has no inner electrons

Answer: B
The additional 3d electrons shield poorly, increasing effective nuclear attraction and causing contraction.

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