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Wednesday, August 19, 2026

Arrange in increasing order of size: Cl−,S2−,Ca2+,Ar.

1. Arrange in increasing order of size: Cl−, S2−, Ca2+, Ar. +
Correct Answer
Ca2+ < Ar < Cl− < S2−
JEE-Level Explanation
First calculate the number of electrons in each species:
Ca2+ = 20 − 2 = 18 e−
Ar = 18 e−
Cl− = 17 + 1 = 18 e−
S2− = 16 + 2 = 18 e−
All four species contain 18 electrons. Therefore, they form an isoelectronic series. For an isoelectronic series:
Nuclear charge ↑  ⇒  Radius ↓
The number of protons is:
Ca2+ Z = 20
18 e−
Ar Z = 18
18 e−
Cl− Z = 17
18 e−
S2− Z = 16
18 e−
Thus Ca2+ has the highest nuclear charge and attracts its 18 electrons most strongly, giving it the smallest radius. S2− has the lowest nuclear charge and therefore the weakest attraction, giving it the largest radius.
Ca2+ < Ar < Cl− < S2−
Quick JEE Trick
When species are isoelectronic, ignore the electron count because it is the same for all. Simply arrange them according to atomic number in the reverse order:
Z: 20 > 18 > 17 > 16

Radius: Ca2+ < Ar < Cl− < S2−
Higher Z = smaller radius for an isoelectronic series.
30 Related JEE-Level MCQs with Solutions
1. Which has the smallest radius among Ca2+, Ar, Cl− and S2−?
A. S2−
B. Cl−
C. Ar
D. Ca2+
Answer: D. Ca2+
2. Which has the largest radius?
A. Ca2+
B. Ar
C. Cl−
D. S2−
Answer: D. S2−
3. All the given species have how many electrons?
A. 16
B. 17
C. 18
D. 20
Answer: C. 18 electrons
4. The given species form a/an:
A. Isotonic series
B. Isoelectronic series
C. Isobaric series
D. Isotopic series
Answer: B. Isoelectronic series
5. Number of protons in Ca2+ is:
A. 18
B. 20
C. 22
D. 16
Answer: B. 20
6. Number of electrons in Cl− is:
A. 16
B. 17
C. 18
D. 19
Answer: C. 18
7. Number of electrons in S2− is:
A. 14
B. 16
C. 18
D. 20
Answer: C. 18
8. Which species has the greatest nuclear charge?
A. S2−
B. Cl−
C. Ar
D. Ca2+
Answer: D. Ca2+
9. For isoelectronic species, increasing atomic number causes radius to:
A. Increase
B. Decrease
C. Remain constant
D. First increase then decrease
Answer: B. Decrease
10. Which has the weakest nuclear attraction for its electrons?
A. Ca2+
B. Ar
C. Cl−
D. S2−
Answer: D. S2−
11. The correct decreasing order of size is:
A. Ca2+ > Ar > Cl− > S2−
B. S2− > Cl− > Ar > Ca2+
C. Ar > S2− > Ca2+ > Cl−
D. Cl− > S2− > Ar > Ca2+
Answer: B
12. Which species is neutral?
A. Ca2+
B. Cl−
C. S2−
D. Ar
Answer: D. Ar
13. Which has the highest effective nuclear attraction?
A. S2−
B. Cl−
C. Ar
D. Ca2+
Answer: D. Ca2+
14. S2− is larger than Cl− because:
A. S has more shells
B. S has lower nuclear charge
C. S has more protons
D. S is a noble gas
Answer: B
15. Cl− and Ar have the same:
A. Number of protons
B. Number of neutrons
C. Number of electrons
D. Atomic number
Answer: C. Number of electrons
16. Ca2+ is smaller than Ar mainly due to:
A. More electrons
B. Greater nuclear charge
C. Fewer protons
D. More shells
Answer: B. Greater nuclear charge
17. The atomic number of S is:
A. 14
B. 16
C. 17
D. 18
Answer: B. 16
18. The atomic number of Cl is:
A. 15
B. 16
C. 17
D. 18
Answer: C. 17
19. The atomic number of Ar is:
A. 16
B. 17
C. 18
D. 20
Answer: C. 18
20. The atomic number of Ca is:
A. 18
B. 19
C. 20
D. 22
Answer: C. 20
21. Which pair has the same number of electrons?
A. S2− and Cl−
B. Ca2+ and Ar
C. Both A and B
D. Neither
Answer: C. Both A and B
22. In an isoelectronic series, radius is inversely related to:
A. Nuclear charge
B. Number of shells only
C. Mass number only
D. Neutron number only
Answer: A. Nuclear charge
23. Which has the greatest number of protons?
A. S2−
B. Cl−
C. Ar
D. Ca2+
Answer: D. Ca2+
24. Which statement is correct for an isoelectronic series?
A. All have identical radii
B. All have identical nuclear charges
C. All have the same number of electrons
D. All are neutral
Answer: C
25. Which species has the most diffuse electron cloud?
A. Ca2+
B. Ar
C. Cl−
D. S2−
Answer: D. S2−
26. Which is the correct increasing order of atomic number?
A. Ca < Ar < Cl < S
B. S < Cl < Ar < Ca
C. Cl < S < Ca < Ar
D. Ar < Ca < S < Cl
Answer: B. S < Cl < Ar < Ca
27. For the given series, which factor is kept constant?
A. Nuclear charge
B. Number of electrons
C. Atomic mass
D. Number of protons
Answer: B. Number of electrons
28. Which species has the highest electron-to-proton ratio?
A. Ca2+
B. Ar
C. Cl−
D. S2−
Answer: D. S2−
29. If nuclear charge increases while electron number remains constant, electron cloud contracts because:
A. Attraction decreases
B. Attraction increases
C. Electrons disappear
D. Shell number doubles
Answer: B. Attraction increases
30. The best shortcut for this question is:
A. Compare atomic masses
B. Compare neutron numbers
C. Identify isoelectronic species and compare Z
D. Compare melting points
Answer: C — Same electrons, so higher Z means smaller radius.
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