Define Covalent Radius and Metallic Radius.

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 1. Covalent Radius

The covalent radius of an atom is defined as half the distance between the nuclei of two identical atoms joined by a single covalent bond.

Covalent Radius = ½ × Internuclear Distance

Atom
A — Atom
A

If the distance between the nuclei of two bonded identical atoms is d, then:

r_c = d / 2

Example: In an H₂ molecule, the distance between the nuclei of the two hydrogen atoms is approximately twice the covalent radius of hydrogen.

🟣 2. Metallic Radius

The metallic radius of an atom is defined as half the distance between the nuclei of two adjacent metal atoms in a metallic crystal.

Metallic Radius = ½ × Distance between adjacent metal nuclei

M — M

If the distance between the nuclei of two adjacent metal atoms is d, then:

r_m = d / 2

The metallic radius is generally used to describe the size of atoms in a metallic crystal lattice.

🟢 Difference Between Covalent and Metallic Radius

Basis	Covalent Radius	Metallic Radius
Meaning	Half the distance between nuclei of two identical covalently bonded atoms	Half the distance between nuclei of two adjacent metal atoms
Found in	Covalent molecules	Metallic crystals
Bond/Arrangement	Covalent bond	Metallic lattice
Symbol	rc	rm

🎬 Radius Concept

Covalent Radius

A ← r → • ← r → A

Total distance = 2r

↓

Radius = Half of internuclear distance

✅ Final Answer

Covalent Radius: It is half the distance between the nuclei of two identical atoms joined by a single covalent bond.

Metallic Radius: It is half the distance between the nuclei of two adjacent metal atoms in a metallic crystal.

r_c = ½ × internuclear distance
r_m = ½ × distance between adjacent metal atoms

📝 20 MCQs with Answers

1. Covalent radius is defined as:

A) Distance between two atoms

B) Half the distance between nuclei of two identical covalently bonded atoms

C) Diameter of an atom

D) Distance between electrons

✅ Answer

B) Half the distance between nuclei of two identical covalently bonded atoms.

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2. Metallic radius is measured in:

A) Ionic molecules

B) Metallic crystals

C) Covalent molecules only

D) Gases only

✅ Answer

B) Metallic crystals

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3. Covalent radius is equal to:

A) d

B) 2d

C) d/2

D) d²

✅ Answer

C) d/2

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4. Metallic radius is equal to:

A) d/2

B) 2d

C) d²

D) d+2

✅ Answer

A) d/2

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5. Covalent radius is associated with:

A) Covalent bond

B) Ionic bond only

C) Metallic bond only

D) Hydrogen bond only

✅ Answer

A) Covalent bond

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6. Metallic radius refers to the size of atoms in:

A) Metallic lattice

B) Water

C) Gas molecules

D) Ionic solution

✅ Answer

A) Metallic lattice

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7. The distance used to determine covalent radius is between:

A) Electron and nucleus

B) Nuclei of bonded atoms

C) Two electrons

D) Two ions in solution

✅ Answer

B) Nuclei of bonded atoms

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8. The distance used to determine metallic radius is between:

A) Nuclei of adjacent metal atoms

B) Electrons only

C) Ions in solution

D) Nucleus and electron

✅ Answer

A) Nuclei of adjacent metal atoms

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9. If internuclear distance in a covalent molecule is 200 pm, its covalent radius is:

A) 50 pm

B) 100 pm

C) 200 pm

D) 400 pm

✅ Answer

B) 100 pm

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10. If adjacent metal nuclei are 300 pm apart, metallic radius is:

A) 50 pm

B) 100 pm

C) 150 pm

D) 300 pm

✅ Answer

C) 150 pm

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11. Covalent radius is generally used for:

A) Covalently bonded atoms

B) Free electrons

C) Metallic ions only

D) Neutrons

✅ Answer

A) Covalently bonded atoms

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12. Metallic radius is generally used for:

A) Metal atoms in a crystal

B) Non-metal molecules only

C) Noble gases only

D) Anions only

✅ Answer

A) Metal atoms in a crystal

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13. Which radius is based on half the internuclear distance?

A) Covalent radius

B) Both covalent and metallic radius

C) Only ionic radius

D) Nuclear radius

✅ Answer

B) Both covalent and metallic radius

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14. The symbol commonly used for covalent radius is:

A) r_c

B) r_m

C) r_i

D) R_g

✅ Answer

A) r_c

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15. The symbol commonly used for metallic radius is:

A) r_c

B) r_m

C) r_i

D) r_a

✅ Answer

B) r_m

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16. Which statement is correct?

A) Covalent radius is half the internuclear distance in a covalent bond

B) Covalent radius is always the full internuclear distance

C) Metallic radius is measured between electrons

D) Metallic radius is the diameter of a metal atom

✅ Answer

A) Covalent radius is half the internuclear distance in a covalent bond.

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17. Metallic radius is especially useful for describing:

A) Crystal structure of metals

B) Molecular mass

C) Boiling point only

D) Atomic number

✅ Answer

A) Crystal structure of metals

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18. Which one involves identical atoms?

A) Standard covalent radius determination

B) Metallic radius determination

C) Both definitions can involve identical neighbouring atoms

D) Neither

✅ Answer

C) Both definitions can involve identical neighbouring atoms.

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19. If metallic radius is 125 pm, distance between adjacent nuclei is:

A) 62.5 pm

B) 125 pm

C) 250 pm

D) 500 pm

✅ Answer

C) 250 pm

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20. Which pair is correctly matched?

A) Covalent radius — metallic crystal

B) Metallic radius — covalent molecule

C) Covalent radius — covalent bond

D) Both — electron radius

✅ Answer

C) Covalent radius — covalent bond

Q1. Define covalent radius.

Solution: Covalent radius is half the distance between the nuclei of two identical atoms joined by a single covalent bond.

Q2. Define metallic radius.

Solution: Metallic radius is half the distance between the nuclei of two adjacent metal atoms in a metallic crystal.

Q1. Differentiate between covalent radius and metallic radius.

Covalent Radius	Metallic Radius
Used for covalently bonded atoms	Used for atoms in metallic crystals
Half the internuclear distance of bonded atoms	Half the distance between adjacent metal nuclei
Associated with covalent bonding	Associated with metallic crystal arrangement

Q1. Explain covalent radius and metallic radius with mathematical expressions.

Solution:

For two identical covalently bonded atoms separated by distance d:

r_c = d/2

For two adjacent metal atoms separated by distance d:

r_m = d/2

Thus, in both cases the radius is obtained by taking half the appropriate distance between the two nuclei.

Q1. Explain different types of atomic radius with special reference to covalent and metallic radius.

Solution:

• Covalent radius: Half the internuclear distance between two identical covalently bonded atoms.
• Metallic radius: Half the distance between the nuclei of two adjacent atoms in a metallic crystal.
• Both are measures of atomic size under different bonding or structural conditions.
• Covalent radius is used mainly for covalent compounds.
• Metallic radius is used for metals in their crystal structures.

Key Point:
Atomic radius depends on the chemical environment in which an atom is present.

Q1. Define covalent and metallic radius. Explain their determination with suitable diagrams and formulae.

Solution:

Atomic size cannot be determined as an absolute boundary because the electron cloud of an atom does not have a sharply defined outer edge. Therefore, different operational definitions of atomic radius are used.

1. Covalent Radius

When two identical atoms are joined by a single covalent bond, the covalent radius is taken as half the distance between their nuclei.

r_c = ½ × internuclear distance

A — d — A

Therefore, d = 2r_c.

2. Metallic Radius

In a metallic crystal, the metallic radius is taken as half the distance between the nuclei of two adjacent metal atoms.

r_m = ½ × distance between adjacent nuclei

M — d — M

The metallic radius is therefore a useful measure of atomic size in a metallic crystal lattice.

Point	Covalent Radius	Metallic Radius
System	Covalent molecule	Metallic crystal
Measurement	½ internuclear distance	½ distance between adjacent metal nuclei
Association	Covalent bond	Metallic lattice

🎯 Final Answer:

Covalent Radius: Half the distance between the nuclei of two identical atoms joined by a single covalent bond.

Metallic Radius: Half the distance between the nuclei of two adjacent metal atoms in a metallic crystal.

🎯 Quick Revision

Covalent Radius → ½ × internuclear distance
Metallic Radius → ½ × distance between adjacent metal nuclei
Covalent Radius → Covalent bond
Metallic Radius → Metallic crystal
Both → Measures of atomic size

Why are s-block elements called "reactive metals"?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 Step 1: What are s-block elements?

The s-block elements are the elements in which the differentiating electron enters the s-subshell. They mainly include Group 1 (alkali metals) and Group 2 (alkaline earth metals).

s-block → Groups 1 and 2

🟢 Step 2: Why are they highly reactive?

s-block metals have only one or two electrons in their outermost shell. These electrons can be removed relatively easily because of their comparatively low ionisation enthalpy.

Li
2s¹ → Li⁺ + e⁻

Therefore, they readily lose their valence electrons and form stable positive ions such as Na⁺, K⁺, Mg²⁺ and Ca²⁺.

🟣 Main Reasons for High Reactivity

Reason	Explanation
1. Few valence electrons	They have only 1 or 2 electrons in the outermost shell.
2. Low ionisation enthalpy	Valence electrons can be removed easily.
3. Large atomic size	The outer electrons are relatively far from the nucleus.
4. Electropositive nature	They readily lose electrons and form cations.

🎬 Electron-Loss Concept

Alkali Metal: Na

Na
2,8,1 → Na⁺
2,8

↓

🎯 One electron is lost easily → High reactivity

🟠 Examples

Group 1: Li, Na, K, Rb, Cs

Group 2: Be, Mg, Ca, Sr, Ba

Among Group 1 elements, reactivity generally increases down the group because atomic size increases and the outer electron becomes easier to remove.

Li < Na < K < Rb < Cs
Reactivity generally increases ↓

✅ Final Answer

s-block elements are called reactive metals because they have only one or two valence electrons and can lose these electrons easily due to their low ionisation enthalpy.

They readily form positive ions and therefore show high chemical reactivity.

📝 20 MCQs with Answers

1. Why are s-block elements highly reactive?

A) They have many valence electrons

B) They easily lose their valence electrons

C) They cannot form ions

D) They have high ionisation enthalpy

✅ Answer

B) They easily lose their valence electrons

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2. Which groups mainly constitute the s-block?

A) Groups 1 and 2

B) Groups 13–18

C) Groups 3–12

D) Groups 15–18

✅ Answer

A) Groups 1 and 2

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3. How many valence electrons does a Group 1 element normally have?

A) 1

B) 2

C) 3

D) 8

✅ Answer

A) 1

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4. How many valence electrons does a Group 2 element normally have?

A) 1

B) 2

C) 6

D) 8

✅ Answer

B) 2

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5. s-block elements generally form:

A) Anions

B) Cations

C) Neutral atoms only

D) Noble gases

✅ Answer

B) Cations

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6. Which of the following is an alkali metal?

A) Na

B) Ca

C) Al

D) Cl

✅ Answer

A) Na

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7. Which of the following is an alkaline earth metal?

A) K

B) Mg

C) Na

D) Cl

✅ Answer

B) Mg

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8. The ionisation enthalpy of alkali metals is generally:

A) High

B) Low

C) Infinite

D) Zero

✅ Answer

B) Low

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9. Which metal is more reactive?

A) Li

B) Na

C) K

D) Rb

✅ Answer

D) Rb

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10. Reactivity of Group 1 elements generally:

A) Decreases down the group

B) Increases down the group

C) Remains constant

D) First decreases then increases

✅ Answer

B) Increases down the group

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11. s-block elements are generally:

A) Electropositive

B) Electronegative

C) Noble gases

D) Metalloids

✅ Answer

A) Electropositive

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12. Sodium forms which ion?

A) Na⁻

B) Na²⁺

C) Na⁺

D) Na³⁺

✅ Answer

C) Na⁺

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13. Calcium forms which ion?

A) Ca⁺

B) Ca²⁺

C) Ca³⁺

D) Ca⁻

✅ Answer

B) Ca²⁺

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14. Which property favours high reactivity of s-block metals?

A) High ionisation enthalpy

B) Low ionisation enthalpy

C) High electron affinity only

D) Complete octet

✅ Answer

B) Low ionisation enthalpy

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15. The outer configuration of Group 1 elements is:

A) ns¹

B) ns²

C) ns²np⁶

D) nd¹

✅ Answer

A) ns¹

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16. The outer configuration of Group 2 elements is:

A) ns¹

B) ns²

C) np²

D) nd²

✅ Answer

B) ns²

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17. Which is the most reactive among the following Group 1 metals?

A) Li

B) Na

C) K

D) Cs

✅ Answer

D) Cs

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18. s-block metals readily react because they tend to achieve:

A) A stable noble-gas configuration

B) More unpaired electrons

C) A half-filled d-orbital

D) A filled f-orbital

✅ Answer

A) A stable noble-gas configuration

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19. Which of the following belongs to Group 2?

A) K

B) Ca

C) Cl

D) Al

✅ Answer

B) Ca

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20. The general electronic configuration of s-block elements is:

A) ns¹⁻²

B) ns²np¹⁻⁶

C) (n−1)d¹⁻¹⁰ns²

D) (n−2)f¹⁻¹⁴ns²

✅ Answer

A) ns¹⁻²

Q1. Why are s-block elements called reactive metals?

Solution: They have only one or two valence electrons and low ionisation enthalpy. Hence, they lose their valence electrons easily and form stable cations.

Q2. What is the general electronic configuration of s-block elements?

Solution:

ns¹⁻²

Group 1 elements have ns¹, while Group 2 elements have ns².

Q1. Give three reasons for the high reactivity of s-block elements.

Solution:

1. They have only 1 or 2 valence electrons.
2. They have relatively low ionisation enthalpy.
3. They readily lose electrons and form stable cations.

Q1. Explain the difference between Group 1 and Group 2 metals with respect to reactivity.

Feature	Group 1	Group 2
Valence electrons	1	2
Configuration	ns¹	ns²
Ion formed	M⁺	M²⁺
General reactivity	Very high	High, but generally lower than Group 1

Q1. Explain why the reactivity of alkali metals increases down the group.

Solution:

• Atomic size increases down the group.
• The outermost electron moves farther from the nucleus.
• Shielding effect also increases.
• Attraction between the nucleus and valence electron decreases.
• Therefore, ionisation enthalpy decreases and electron loss becomes easier.

Li → Na → K → Rb → Cs
Reactivity generally increases ↓

Q1. Explain in detail why s-block elements are called reactive metals.

Solution:

s-block elements comprise mainly the elements of Groups 1 and 2. Their general outer electronic configuration is ns¹⁻². Because they possess only one or two valence electrons, they can lose these electrons comparatively easily.

Factor	Effect
Few valence electrons	Easy electron loss
Low ionisation enthalpy	Electrons are removed easily
Large atomic size	Valence electrons are farther from nucleus
Electropositive character	Stable cations are formed

For example, sodium has the electronic configuration 2,8,1. It readily loses one electron to form Na⁺, which has the stable configuration 2,8.

Na → Na⁺ + e⁻

Thus, s-block metals readily participate in chemical reactions and are therefore called reactive metals.

🎯 Final Answer:

s-block elements are called reactive metals because their one or two valence electrons are easily lost due to relatively low ionisation enthalpy, resulting in the formation of stable positive ions.

🎯 Quick Revision

s-block → Groups 1 & 2
General configuration → ns¹⁻²
Valence electrons → 1 or 2
Nature → Electropositive
Ion formation → Cations
Reason for reactivity → Easy loss of valence electrons
Group 1 reactivity → Increases down the group

What is the characteristic configuration of f-block elements?

📚 Chapter: Classification of Elements and Periodicity in Properties

📘 Detailed Solution

🔵 Step 1: What are f-block elements?

The f-block elements are those elements in which the differentiating electron enters the (n−2)f subshell. They are placed separately at the bottom of the periodic table.

Differentiating electron → (n−2)f subshell

🟢 Step 2: General Electronic Configuration

The general or characteristic electronic configuration of f-block elements is:

(n−2)f^1–14(n−1)d^0–1ns²

Here, the (n−2)f subshell is progressively filled from one element to the next.

f¹ f² f³ → f¹⁴

🟣 Two Series of f-Block Elements

Series	Subshell Being Filled	Period
Lanthanoids	4f	6th
Actinoids	5f	7th

Lanthanoids → 4f^1–14
Actinoids → 5f^1–14

🎬 f-Orbital Filling

(n−2)f^1–14

f¹ f² f³ ... f¹⁴

↓

🎯 f-subshell is progressively filled

🟠 Important Examples

Lanthanoids: Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu

Actinoids: Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr

The f-block elements are also called inner transition elements.

✅ Final Answer

The characteristic electronic configuration of f-block elements is:

(n−2)f^1–14(n−1)d^0–1ns²

The differentiating electron enters the (n−2)f subshell.

📝 20 MCQs with Answers

1. What is the characteristic configuration of f-block elements?

A) ns¹

B) ns²np⁶

C) (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²

D) (n−1)d¹⁻¹⁰ns²

✅ Answer

C) (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²

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2. In f-block elements, the differentiating electron enters the:

A) ns orbital

B) np orbital

C) (n−1)d orbital

D) (n−2)f orbital

✅ Answer

D) (n−2)f orbital

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3. f-block elements are also known as:

A) Representative elements

B) Inner transition elements

C) Noble gases

D) Halogens

✅ Answer

B) Inner transition elements

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4. Lanthanoids involve filling of which subshell?

A) 3d

B) 4f

C) 5d

D) 5f

✅ Answer

B) 4f

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5. Actinoids involve filling of which subshell?

A) 4f

B) 4d

C) 5f

D) 6d

✅ Answer

C) 5f

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6. How many electrons can an f-subshell accommodate?

A) 2

B) 6

C) 10

D) 14

✅ Answer

D) 14

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7. Lanthanoids belong mainly to the:

A) 4th period

B) 5th period

C) 6th period

D) 7th period

✅ Answer

C) 6th period

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8. Actinoids belong mainly to the:

A) 5th period

B) 6th period

C) 7th period

D) 4th period

✅ Answer

C) 7th period

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9. Which of the following is a lanthanoid?

A) U

B) Ce

C) Th

D) Ac

✅ Answer

B) Ce

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10. Which of the following is an actinoid?

A) La

B) Ce

C) U

D) Eu

✅ Answer

C) U

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11. The f-block is placed separately at the:

A) Top of periodic table

B) Bottom of periodic table

C) Centre of periodic table

D) Right side only

✅ Answer

B) Bottom of periodic table

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12. The maximum number of electrons in f-subshell is:

A) 6

B) 10

C) 14

D) 18

✅ Answer

C) 14

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13. General configuration of lanthanoids involves:

A) 3f

B) 4f

C) 5f

D) 6f

✅ Answer

B) 4f

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14. General configuration of actinoids involves:

A) 3f

B) 4f

C) 5f

D) 6f

✅ Answer

C) 5f

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15. Which series contains uranium?

A) Lanthanoids

B) Actinoids

C) Transition elements

D) Halogens

✅ Answer

B) Actinoids

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16. Which series contains europium?

A) Actinoids

B) Lanthanoids

C) Transition elements

D) Alkali metals

✅ Answer

B) Lanthanoids

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17. f-block elements are mainly:

A) Metals

B) Non-metals

C) Noble gases

D) Metalloids

✅ Answer

A) Metals

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18. The f-block contains:

A) Only lanthanoids

B) Only actinoids

C) Lanthanoids and actinoids

D) Halogens

✅ Answer

C) Lanthanoids and actinoids

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19. The f-subshell has how many orbitals?

A) 3

B) 5

C) 7

D) 9

✅ Answer

C) 7

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20. Each f-orbital can accommodate:

A) 1 electron

B) 2 electrons

C) 4 electrons

D) 6 electrons

✅ Answer

B) 2 electrons

Q1. Write the general electronic configuration of f-block elements.

Solution:

(n−2)f^1–14(n−1)d^0–1ns²

Q2. Which subshell is progressively filled in f-block elements?

Solution: The (n−2)f subshell is progressively filled.

Q1. Explain the characteristic configuration of f-block elements.

Solution:

1. The differentiating electron enters the (n−2)f subshell.
2. The general configuration is (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns².
3. They include the lanthanoids and actinoids.

Q1. Differentiate between lanthanoids and actinoids on the basis of electronic configuration.

Feature	Lanthanoids	Actinoids
Subshell filled	4f	5f
Period	6th	7th
General filling	4f¹⁻¹⁴	5f¹⁻¹⁴

Q1. Explain the position and electronic configuration of f-block elements.

Solution:

• f-block elements are placed separately at the bottom of the periodic table.
• They are called inner transition elements.
• The differentiating electron enters the (n−2)f subshell.
• Their general configuration is (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns².
• They are divided into lanthanoids (4f) and actinoids (5f).

f-block → (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²

Q1. Explain in detail the characteristic electronic configuration of f-block elements and their two series.

Solution:

The f-block elements are those in which the differentiating electron enters the (n−2)f subshell. They are known as inner transition elements and are placed separately at the bottom of the periodic table.

(n−2)f^1–14(n−1)d^0–1ns²

There are two series:

Series	Subshell Filled	Period
Lanthanoids	4f¹⁻¹⁴	6th
Actinoids	5f¹⁻¹⁴	7th

The f-subshell contains seven orbitals and can accommodate a maximum of 14 electrons. Therefore, the f-block configuration is represented by f¹ to f¹⁴.

🎯 Final Answer:

Characteristic configuration of f-block elements:

(n−2)f^1–14(n−1)d^0–1ns²

The differentiating electron enters the (n−2)f subshell.

🎯 Quick Revision

f-block → Inner Transition Elements
General Configuration → (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns²
Differentiating Electron → (n−2)f
Lanthanoids → 4f
Actinoids → 5f
Maximum f-electrons → 14