Define Isoelectronic species with two examples.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Explanation

🔵 Trend Down a Group

The ionic radius generally increases down a group in the periodic table.

⬇️ Down the Group → Ionic Radius Increases ⬆️

This happens because a new electron shell is added as we move from one period to the next. The outermost electrons are therefore located farther from the nucleus.

🟣 Main Reasons

1. Increase in number of shells: Each successive element down a group has an additional principal energy level.
2. Increase in shielding effect: Inner electrons shield the valence electrons from the attraction of the nucleus.
3. Greater distance from nucleus: The outermost electron shell lies farther away from the nucleus.

More shells + More shielding → Larger ionic radius

🟢 Example: Group 1 Cations

Consider the alkali metal ions:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

Ionic radius increases from Li⁺ → Cs⁺.

⚡ Important Point: Cations and Anions

Type	Effect
Cation (+)	Smaller than its parent atom
Anion (−)	Larger than its parent atom

However, within a particular group, both cationic and anionic radii generally increase as we move downward because additional electron shells are added.

✅ Final Answer

The ionic radius increases down a group because the number of electron shells increases, resulting in greater shielding and a greater distance between the nucleus and the outermost electron shell.

Down the group ↓ → Ionic radius ↑

📝 20 MCQs with Answers

1. What happens to ionic radius down a group?

A) Decreases

B) Increases

C) Remains constant

D) Becomes zero

✅ Answer

B) Increases

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2. The main reason for increasing ionic radius down a group is:

A) Decrease in shells

B) Addition of electron shells

C) Decrease in nuclear charge

D) Loss of protons

✅ Answer

B) Addition of electron shells

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3. Shielding effect down a group generally:

A) Decreases

B) Increases

C) Becomes zero

D) Remains exactly constant

✅ Answer

B) Increases

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4. Which has the larger ionic radius?

A) Li⁺

B) Na⁺

C) Both are equal

D) Cannot be compared

✅ Answer

B) Na⁺

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5. Correct order of Group 1 ionic radii is:

A) Li⁺ > Na⁺ > K⁺

B) Li⁺ < Na⁺ < K⁺

C) K⁺ < Na⁺ < Li⁺

D) Na⁺ < Li⁺ < K⁺

✅ Answer

B) Li⁺ < Na⁺ < K⁺

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6. Down a group, the number of occupied shells:

A) Decreases

B) Increases

C) Remains constant

D) Becomes zero

✅ Answer

B) Increases

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7. Which factor increases the distance of the outermost electrons from the nucleus?

A) Addition of electron shells

B) Removal of shells

C) Decrease in atomic number

D) Nuclear contraction only

✅ Answer

A) Addition of electron shells

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8. Which statement is correct?

A) Ionic radius always decreases down every group

B) Ionic radius generally increases down a group

C) Ionic radius is independent of shells

D) Ionic radius is always constant

✅ Answer

B) Ionic radius generally increases down a group

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9. A cation is generally ______ than its parent atom.

A) Larger

B) Smaller

C) Equal

D) Heavier only

✅ Answer

B) Smaller

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10. An anion is generally ______ than its parent atom.

A) Smaller

B) Larger

C) Equal

D) Without electrons

✅ Answer

B) Larger

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11. Which ion is expected to have the largest radius?

A) Li⁺

B) Na⁺

C) K⁺

D) Rb⁺

✅ Answer

D) Rb⁺

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12. What is the effect of inner electrons on outer electrons?

A) Shielding

B) Bonding

C) Ionisation

D) Fusion

✅ Answer

A) Shielding

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13. Which quantity generally increases down a group?

A) Ionic radius

B) Nuclear attraction on outermost shell

C) Electronegativity

D) Ionisation energy

✅ Answer

A) Ionic radius

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14. The outermost electron shell becomes:

A) Closer to nucleus down a group

B) Farther from nucleus down a group

C) Absent

D) Identical for all elements

✅ Answer

B) Farther from nucleus down a group

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15. Which pair represents a correct trend?

A) Down group → radius decreases

B) Down group → radius increases

C) Down group → shells decrease

D) Down group → shielding decreases

✅ Answer

B) Down group → radius increases

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16. Ionic radius is a measure of:

A) Size of an ion

B) Mass of an ion

C) Number of neutrons only

D) Melting point

✅ Answer

A) Size of an ion

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17. Which has a greater ionic radius?

A) K⁺

B) Li⁺

C) Both equal

D) H⁺

✅ Answer

A) K⁺

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18. The shielding effect is caused mainly by:

A) Inner-shell electrons

B) Protons only

C) Neutrons only

D) Nucleus only

✅ Answer

A) Inner-shell electrons

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19. Which sequence shows increasing ionic radius?

A) Li⁺ < Na⁺ < K⁺

B) K⁺ < Na⁺ < Li⁺

C) Na⁺ < Li⁺ < K⁺

D) Li⁺ > K⁺ > Na⁺

✅ Answer

A) Li⁺ < Na⁺ < K⁺

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20. The best explanation for increasing ionic radius down a group is:

A) More electron shells and greater shielding

B) Fewer electron shells

C) Fewer protons

D) Complete removal of electrons

✅ Answer

A) More electron shells and greater shielding

Q. State the trend of ionic radii down a group.

Solution: Ionic radius generally increases down a group because a new electron shell is added at each successive period. The increased shielding effect also places the outermost electrons farther from the nucleus.

Down the group ↓ → Ionic radius ↑

Q. Why does ionic radius increase down a group?

Solution:

1. The number of occupied electron shells increases down the group.
2. The shielding effect of inner electrons increases.
3. The outermost electrons become farther from the nucleus, so the ionic radius increases.

Therefore: Ionic radius increases down a group.

Q. Explain the variation of ionic radius down a group with an example.

Solution:

Ion	Relative Size	Reason
Li⁺	Smallest	Fewer electron shells
Na⁺	↑	One additional shell
K⁺	↑↑	More shells and shielding
Rb⁺	↑↑↑	Still more shells

Li⁺ < Na⁺ < K⁺ < Rb⁺

Q. Discuss the trend of ionic radius down a group and explain the factors responsible for it.

Solution:

Ionic radius generally increases as we move down a group in the periodic table. The following factors are responsible:

• Increase in principal quantum number: Each step down adds a new electron shell.
• Increase in shielding effect: Inner electrons increasingly shield the outer electrons from the nucleus.
• Increase in distance: The outermost electron shell lies farther from the nucleus.
• Reduced effective attraction: Although nuclear charge increases, the additional shells and shielding cause the outer electrons to experience weaker effective attraction.
• Hence, the overall ionic size increases down the group.

General Trend:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

Q. Explain in detail the trend of ionic radii down a group. Also explain the role of electron shells and shielding effect with a suitable example.

Detailed Solution:

The ionic radius is the effective size of an ion. It depends mainly on the number of occupied electron shells and the attraction between the nucleus and the electrons.

When we move down a group, the atomic number increases and a new principal energy level is added at each step. Therefore, the outermost electrons are located farther away from the nucleus.

Factor	Effect Down the Group
Number of shells	Increases
Shielding effect	Increases
Distance of outer shell from nucleus	Increases
Ionic radius	Increases

Example: Group 1 Cations

Li⁺ → Na⁺ → K⁺ → Rb⁺ → Cs⁺

Ionic radius increases from Li⁺ to Cs⁺.

For example, Li⁺ has electrons only in the first shell, while Na⁺ has electrons up to the second shell and K⁺ has electrons up to the third shell. Thus, the outer electron shell is progressively farther from the nucleus.

The inner-shell electrons also shield the outer electrons from the full positive charge of the nucleus. This shielding effect increases down the group and contributes to the increase in ionic size.

🎯 Final Answer:

The ionic radius generally increases down a group because successive elements have additional electron shells and greater shielding effect. These factors increase the distance of the outermost electrons from the nucleus, resulting in a larger ionic radius.

Down the group ↓ → Number of shells ↑ → Shielding ↑ → Ionic radius ↑

🎯 Quick Revision

Down a Group ↓
Electron shells ↑
Shielding effect ↑
Distance from nucleus ↑
Ionic Radius ↑

Example:
Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺