1. Which has the smallest radius:
O2−, F−, Na+, Mg2+?
+
Answer
Mg2+ — Smallest radius
Detailed Explanation
All four species are isoelectronic, meaning they contain the
same number of electrons.
As the number of protons increases, the nucleus attracts the same number of electrons more strongly. Therefore, the ionic radius decreases.
Therefore, the decreasing order of radius is:
O2− = 8 + 2 = 10 e−
F− = 9 + 1 = 10 e−
Na+ = 11 − 1 = 10 e−
Mg2+ = 12 − 2 = 10 e−
Since all have 10 electrons, their sizes are mainly determined
by the nuclear charge.
F− = 9 + 1 = 10 e−
Na+ = 11 − 1 = 10 e−
Mg2+ = 12 − 2 = 10 e−
As the number of protons increases, the nucleus attracts the same number of electrons more strongly. Therefore, the ionic radius decreases.
Z ↑ ⇒ Nuclear attraction ↑ ⇒ Radius ↓
The nuclear charges are:
O2−
Z = 8
F−
Z = 9
Na+
Z = 11
Mg2+
Z = 12
Smallest
Smallest
O2− > F− > Na+ > Mg2+
Quick Trick for Isoelectronic Species
When species have the same number of electrons:
Hence, Mg2+ has the strongest nuclear attraction and the smallest radius.
Higher atomic number → Smaller radius
So simply compare the number of protons:
O (8) < F (9) < Na (11) < Mg (12)
Hence, Mg2+ has the strongest nuclear attraction and the smallest radius.
30 Related MCQs with Solutions
1. Which has the smallest radius among O2−, F−,
Na+ and Mg2+?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: D. Mg2+
2. O2−, F−, Na+ and Mg2+
are:
A. Isotopes
B. Isobars
C. Isoelectronic species
D. Isotones
Answer: C. Isoelectronic species
3. Number of electrons in Mg2+ is:
A. 8
B. 10
C. 12
D. 14
Answer: B. 10
4. Number of electrons in O2− is:
A. 6
B. 8
C. 10
D. 12
Answer: C. 10
5. Number of electrons in F− is:
A. 8
B. 9
C. 10
D. 11
Answer: C. 10
6. Number of electrons in Na+ is:
A. 9
B. 10
C. 11
D. 12
Answer: B. 10
7. In an isoelectronic series, increasing nuclear charge generally:
A. Increases radius
B. Decreases radius
C. Has no effect
D. Doubles radius
Answer: B. Decreases radius
8. Which has the largest radius?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: A. O2−
9. The correct decreasing order of radius is:
A. Mg2+ > Na+ > F− > O2−
B. O2− > F− > Na+ > Mg2+
C. F− > O2− > Mg2+ > Na+
D. Na+ > Mg2+ > O2− > F−
Answer: B
10. Which ion has the greatest nuclear charge in this isoelectronic
series?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: D. Mg2+
11. Which factor primarily determines the size in an isoelectronic
series?
A. Number of neutrons
B. Nuclear charge
C. Mass number only
D. Melting point
Answer: B. Nuclear charge
12. Which species has 10 electrons and 12 protons?
A. Na+
B. Mg2+
C. F−
D. O2−
Answer: B. Mg2+
13. Which species has 8 protons and 10 electrons?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: A. O2−
14. Cations are generally smaller than their parent atoms because:
A. They gain electrons
B. They lose electrons
C. Their nuclear charge decreases
D. Their mass becomes zero
Answer: B. They lose electrons
15. Anions are generally larger than their parent atoms because:
A. They gain electrons
B. They lose electrons
C. They lose protons
D. They lose neutrons
Answer: A. They gain electrons
16. Which is smaller?
A. Na atom
B. Na+ ion
C. Both equal
D. Cannot compare
Answer: B. Na+
17. Which has stronger effective nuclear attraction among these
isoelectronic species?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: D. Mg2+
18. Which pair is isoelectronic?
A. Na+ and Ne
B. Na and Ne
C. Na and F
D. Mg and O
Answer: A. Na+ and Ne
Both contain 10 electrons.
Both contain 10 electrons.
19. Which isoelectronic species has the smallest size?
A. N3−
B. O2−
C. F−
D. Na+
Answer: D. Na+
Na+ has the highest nuclear charge among these four.
Na+ has the highest nuclear charge among these four.
20. The radius of an isoelectronic species decreases as:
A. Atomic number decreases
B. Atomic number increases
C. Electron number increases
D. Neutron number increases
Answer: B. Atomic number increases
21. Which of the following contains 10 electrons?
A. Mg2+
B. Cl−
C. K+
D. All of these
Answer: A. Mg2+
Mg2+ has 10 electrons; Cl− and K+ have 18 electrons.
Mg2+ has 10 electrons; Cl− and K+ have 18 electrons.
22. The effective nuclear charge experienced by electrons is greatest
for:
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: D. Mg2+
23. Which species is largest among N3−, O2−,
F− and Na+?
A. N3−
B. O2−
C. F−
D. Na+
Answer: A. N3−
24. Which of the following represents the correct rule for an
isoelectronic series?
A. Higher Z → Larger radius
B. Higher Z → Smaller radius
C. Higher Z → Same radius
D. Z has no effect
Answer: B. Higher Z → Smaller radius
25. Which ion has the greatest positive charge?
A. O2−
B. F−
C. Na+
D. Mg2+
Answer: D. Mg2+
26. Which species has the same electronic configuration as Ne?
A. O2−
B. F−
C. Na+
D. All of these
Answer: D. All of these
Each contains 10 electrons.
Each contains 10 electrons.
27. Mg2+ has fewer electrons than Mg atom by:
A. 1
B. 2
C. 3
D. 4
Answer: B. 2 electrons
28. Which statement is correct for cation size?
A. Cation is always larger than atom
B. Cation is generally smaller than atom
C. Cation and atom always have equal radii
D. Cation has no nucleus
Answer: B
29. Which concept is most useful for comparing O2−,
F−, Na+ and Mg2+?
A. Isoelectronic relationship
B. Isotopic relationship
C. Hydrogen bonding
D. Metallic bonding
Answer: A. Isoelectronic relationship
30. The correct increasing order of ionic radius is:
A. O2− < F− < Na+ < Mg2+
B. Mg2+ < Na+ < F− < O2−
C. F− < O2− < Mg2+ < Na+
D. Na+ < Mg2+ < O2− < F−
Answer: B
For isoelectronic species, higher nuclear charge means smaller radius.
For isoelectronic species, higher nuclear charge means smaller radius.
Advertisement
