Questions on Ionic Radii
Multiple-Choice Questions: Ionic Radii
🔹 Basic Understanding
1. What is ionic radius?
A) The radius of a nucleus
B) The radius of an electron
C) The distance between two ions
D) The effective size of an ion in a crystal lattice
E) The distance from the nucleus to the first proton
2. Compared to their neutral atoms, cations are:
A) Larger
B) Smaller
C) Same size
D) Variable in size
E) Unstable
3. Compared to their neutral atoms, anions are:
A) Smaller
B) Larger
C) Same size
D) Radioactive
E) Positively charged
4. Why are cations smaller than their neutral atoms?
A) Because they gain energy
B) Because protons are removed
C) Because they lose electrons and have less electron repulsion
D) Because they gain neutrons
E) Because the nucleus expands
5. Why are anions larger than their neutral atoms?
A) Because they gain protons
B) Because they gain electrons, increasing repulsion
C) Because the nucleus gets larger
D) Because they form ionic bonds
E) Because they are metals
🔹 Trends Across Periods and Groups
6. As you move down a group, the ionic radius:
A) Decreases
B) Increases
C) Remains constant
D) First increases, then decreases
E) Depends on the ion's charge
7. Which of the following ions has the largest ionic radius?
A) Na⁺
B) Mg²⁺
C) K⁺
D) Ca²⁺
E) Al³⁺
8. Which of the following anions is the smallest?
A) F⁻
B) Cl⁻
C) Br⁻
D) I⁻
E) At⁻
9. Which of the following ions is smaller than its corresponding atom?
A) Cl⁻
B) O²⁻
C) F⁻
D) Na⁺
E) S²⁻
10. What happens to ionic radius across a period (left to right) for cations?
A) Increases
B) Decreases
C) Remains the same
D) First increases, then decreases
E) No trend
🔹 Advanced Comparison
11. Which has the smallest radius?
A) Na
B) Na⁺
C) K⁺
D) Mg²⁺
E) Al³⁺
12. Which has the largest radius?
A) O
B) O²⁻
C) F
D) F⁻
E) Ne
13. Which ion has the same electron configuration as neon?
A) Na⁺
B) O²⁻
C) F⁻
D) All of the above
E) None of the above
14. Which of the following represents an isoelectronic series?
A) O, O²⁻, F
B) Na⁺, Mg²⁺, Al³⁺
C) K⁺, Rb⁺, Cs⁺
D) N³⁻, O²⁻, F⁻
E) He, Ne, Ar
15. Among isoelectronic ions, the one with the highest nuclear charge will be:
A) Largest in size
B) Most reactive
C) Smallest in size
D) Least stable
E) Radioactive
🔹 Real Applications
16. Which ion is most likely to fit tightly into a crystal lattice?
A) Na⁺
B) Cl⁻
C) Mg²⁺
D) F⁻
E) Cs⁺
17. The ionic radius of Al³⁺ is smaller than that of Mg²⁺ because:
A) Al³⁺ has more electrons
B) Al³⁺ has a higher nuclear charge
C) Al³⁺ is a metal
D) Al³⁺ has a larger atomic mass
E) Al³⁺ has more shells
18. When a neutral atom becomes a cation, its electron cloud:
A) Expands significantly
B) Contracts
C) Disappears
D) Doubles in size
E) Gains mass
19. Which ion has the largest radius among the isoelectronic ions?
A) N³⁻
B) O²⁻
C) F⁻
D) Na⁺
E) Mg²⁺
20. Why do anions have lower effective nuclear charge per electron?
A) They have fewer protons
B) They have larger nuclei
C) They have more electrons repelling each other
D) They are always metals
E) They have fewer neutrons
Answers and Explanations
1. D – Ionic radius is the size of an ion as it exists in a crystal or ionic compound.
2. B – Cations are smaller because they have fewer electrons and less repulsion.
3. B – Anions are larger because they have more electrons and increased repulsion.
4. C – Loss of electrons in cations leads to reduced electron-electron repulsion and a smaller size.
5. B – Extra electrons in anions increase electron-electron repulsion, enlarging the radius.
6. B – More shells are added down a group, increasing ionic radius.
7. C – K⁺ has more shells than Na⁺ or Mg²⁺, making it the largest ion here.
8. A – F⁻ is at the top of the group and has the smallest radius among halide anions.
9. D – Cations (like Na⁺) are always smaller than their neutral atoms.
10. B – Across a period, cations have increasing nuclear charge, pulling electrons closer.
11. E – Al³⁺ has the fewest electrons and the highest nuclear charge among these, making it the smallest.
12. B – O²⁻ has gained two electrons, increasing repulsion and size.
13. D – Na⁺, O²⁻, and F⁻ all have 10 electrons, matching neon’s configuration.
14. D – These all have 10 electrons (N³⁻, O²⁻, F⁻) and are isoelectronic.
15. C – Greater nuclear charge in isoelectronic ions pulls electrons closer, reducing radius.
16. C – Mg²⁺ is small and has a high charge, ideal for tight lattice structures.
17. B – Al³⁺ has a higher charge and more nuclear pull, making it smaller than Mg²⁺.
18. B – Cations contract due to stronger pull on fewer electrons.
19. A – N³⁻ has the most electrons and lowest nuclear charge, making it the largest.
20. C – More electrons cause greater repulsion and decrease the effective nuclear pull per electron.
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