📘 Detailed Explanation
🔵 Basic Comparison
The size of an atom changes when it gains or loses electrons. Therefore, the radius of a neutral atom is different from the radius of its cation or anion.
Positive ion < Neutral atom < Negative ion
🔴 Cation – Smaller than Neutral Atom
A cation is formed when a neutral atom loses one or more electrons.
After losing electrons, the number of electrons decreases while the nuclear charge remains the same. In many cases, the outermost shell is completely lost. Hence, the remaining electrons are attracted more strongly towards the nucleus.
Example:
Na → Na⁺ + e⁻
The sodium atom is larger than the sodium ion:
🟢 Anion – Larger than Neutral Atom
An anion is formed when a neutral atom gains one or more electrons.
The nuclear charge remains unchanged, but the number of electrons increases. This increases electron-electron repulsion and reduces the effective attraction experienced by each electron. Therefore, the electron cloud expands.
Example:
Cl + e⁻ → Cl⁻
The chloride ion is larger than the chlorine atom:
📊 Comparison Table
| Species | Electron Change | Effect on Size | Reason |
|---|---|---|---|
| Cation (+) | Loss of electrons | Smaller | Greater effective attraction and often loss of outer shell |
| Neutral Atom | No gain/loss | Reference size | Normal balance of nuclear attraction and electron repulsion |
| Anion (−) | Gain of electrons | Larger | Greater electron-electron repulsion |
🔬 Example with Isoelectronic Species
Consider Na⁺, Ne and F⁻. All three species have 10 electrons, but their nuclear charges are different.
Na⁺ , Ne , F⁻
Na⁺ has the greatest nuclear charge among these three and therefore has the smallest radius, while F⁻ has the lowest nuclear charge and the largest radius.
✅ Final Answer
A cation is smaller than its neutral atom because it loses electrons, whereas an anion is larger because it gains electrons and experiences increased electron-electron repulsion.
📝 20 MCQs with Answers
1. Which is generally the smallest?
✅ Answer
C) Cation
2. A cation is formed by:
✅ Answer
B) Loss of electrons
3. An anion is generally ______ than its neutral atom.
✅ Answer
B) Larger
4. A cation is generally ______ than its neutral atom.
✅ Answer
B) Smaller
5. Which sequence is generally correct?
✅ Answer
B) Cation < Atom < Anion
6. What happens to the number of electrons when a cation forms?
✅ Answer
B) Decreases
7. What happens to the number of electrons when an anion forms?
✅ Answer
B) Increases
8. Which ion is smaller than Na?
✅ Answer
A) Na⁺
9. Which ion is larger than Cl?
✅ Answer
B) Cl⁻
10. Formation of an anion increases:
✅ Answer
A) Electron-electron repulsion
11. During ion formation, the number of protons generally:
✅ Answer
B) Remains unchanged
12. Which has greater electron-electron repulsion?
✅ Answer
A) Anion
13. The radius of Na⁺ is smaller than Na because:
✅ Answer
B)
14. The radius of Cl⁻ is larger than Cl because:
✅ Answer
B) It gains an electron
15. Which species has a positive charge?
✅ Answer
B) Cation
16. Which species has a negative charge?
✅ Answer
B) Anion
17. Which has the largest radius in the simple comparison?
✅ Answer
C) Anion
18. Correct order for size is:
✅ Answer
B) Cl⁺ < Cl < Cl⁻
19. Which statement is correct?
✅ Answer
C)
20. The correct general relationship is:
✅ Answer
B) Anion > Atom > Cation
Q. Compare the size of a neutral atom with its cation and anion.
Solution:
- A cation is smaller than its neutral atom because it loses electrons.
- An anion is larger than its neutral atom because it gains electrons, increasing electron-electron repulsion.
Q. Explain why cations are smaller and anions are larger than their parent atoms.
Solution:
- Cation: Loss of electrons reduces electron-electron repulsion and may remove the outermost shell, making the ion smaller.
- Anion: Gain of electrons increases electron-electron repulsion, causing expansion of the electron cloud.
- The number of protons does not change during ordinary ion formation.
Q. Differentiate between the size of a neutral atom, cation and anion.
| Species | Electron Change | Relative Size | Main Reason |
|---|---|---|---|
| Cation | Loses e⁻ | Smaller | Greater effective attraction; possible loss of outer shell |
| Neutral Atom | No change | Normal | Reference size |
| Anion | Gains e⁻ | Larger | Increased electron-electron repulsion |
Q. Explain the change in atomic size when an atom forms a cation or an anion.
Solution:
Formation of Cation
When an atom loses one or more electrons, a positive ion called a cation is formed. The number of protons remains unchanged, but the number of electrons decreases. The remaining electrons experience stronger attraction from the nucleus. If the complete outer shell is lost, the decrease in size is even more significant.
Formation of Anion
When an atom gains one or more electrons, a negative ion called an anion is formed. The nuclear charge remains unchanged, but electron-electron repulsion increases. Consequently, the electron cloud expands and the ionic radius becomes larger.
Q. Compare the size of a neutral atom, its cation and anion. Explain the reasons with suitable examples.
Detailed Solution
The size of an atom or ion is expressed in terms of its atomic or ionic radius. Formation of an ion involves the loss or gain of electrons, while the number of protons remains unchanged. This produces a significant change in size.
1. Cation
A cation is formed by the loss of electrons. Since the nuclear charge remains the same but fewer electrons are present, the attraction between the nucleus and the remaining electrons becomes stronger. In some cases, the entire outermost shell is removed.
Na⁺ is smaller than Na.
2. Neutral Atom
A neutral atom has an equal number of protons and electrons. Its size lies between the sizes of its corresponding cation and anion in the simple comparison.
3. Anion
An anion is formed by gaining electrons. Since the nuclear charge remains unchanged but the number of electrons increases, electron-electron repulsion increases. The electron cloud expands and the ionic radius increases.
Cl⁻ is larger than Cl.
📊 Overall Comparison
| Species | Electron Change | Size |
|---|---|---|
| Cation (+) | Loss | Smallest |
| Neutral Atom | No change | Intermediate |
| Anion (−) | Gain | Largest |
A cation is generally smaller than its neutral atom because it loses electrons and experiences stronger effective nuclear attraction. An anion is generally larger because it gains electrons, increasing electron-electron repulsion and expanding the electron cloud.
Cation < Neutral Atom < Anion
🎯 Quick Revision
Neutral Atom → Normal Size
Gain e⁻ → Anion → Size ↑
Cation < Neutral Atom < Anion
