Chemical Bonding: Octet Rule, Covalent Bonds & Lewis Structures | NEET

 

KÖssel–Lewis Approach to Chemical Bonding — NEET  Notes

Class 11 CBSE Chemistry: Chemical Bonding and Molecular Structure – Complete Question Bank

1. Basic idea

• Kössel and Lewis independently explained chemical bonding in 1916.
• They were among the first to give a logical explanation of valency.
• Their explanation was based on the stability/inertness of noble gases.
• Main idea: Atoms form chemical bonds to achieve a stable electronic configuration, generally similar to that of a noble gas.

⭐ NEET Key Point

Atoms tend to achieve a stable outer shell, usually an octet of 8 electrons.

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2. Lewis's picture of an atom

• Lewis divided an atom conceptually into two parts:

(a) Kernel

• The kernel consists of:
  • nucleus
  • inner-shell electrons
• These inner electrons generally do not participate in bonding.

(b) Outer shell

• The outermost shell contains the valence electrons.
• Lewis imagined a maximum of 8 electrons in the outer shell.
• He represented these 8 positions as the 8 corners of a cube surrounding the kernel.

Example: Sodium

• Sodium has electronic configuration:

Na = 2, 8, 1

• It has 1 valence electron.
• Therefore, only one corner of Lewis's imaginary cube is occupied.

Noble gas

• A noble gas generally has 8 electrons in its outer shell.
• Therefore, all 8 positions are occupied.
• This represents a very stable arrangement.

⭐ Remember

Noble gas → stable → full outer shell

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3. Octet rule

• Lewis proposed that atoms tend to achieve a stable octet (8 electrons) when they form chemical bonds.
• Atoms can achieve this mainly by:
  1. Losing electrons
  2. Gaining electrons
  3. Sharing electrons

Example: NaCl

• Sodium has 1 valence electron.
• Chlorine has 7 valence electrons.
• Sodium loses 1 electron.
• Chlorine gains 1 electron.
• Both achieve stable noble-gas-like configurations.

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Then:

Na⁺ + Cl⁻ → NaCl

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4. Ionic bonding according to Lewis

• When an electron is completely transferred from one atom to another, ions are formed.
• The atom losing an electron becomes a positive ion (cation).
• The atom gaining an electron becomes a negative ion (anion).
• The oppositely charged ions attract each other through electrostatic attraction.
• This attraction produces an ionic bond.

Example: NaCl

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na⁺ + Cl⁻ → NaCl

⭐ Important

Electron transfer → ions → electrostatic attraction → ionic bond

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5. Covalent bonding according to Lewis

• Not every atom forms a bond by electron transfer.
• In molecules such as:
  • H₂
  • Cl₂
  • F₂
• Atoms form bonds by sharing electrons.

Example: Cl₂

• Each chlorine atom has 7 valence electrons.
• Each chlorine needs 1 more electron to complete its octet.
• Two chlorine atoms share one pair of electrons.
• Therefore, each chlorine effectively gets 8 electrons in its valence shell.

Cl + Cl → Cl₂

⭐ Key idea

Sharing of electron pair → covalent bond

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6. Valence electrons

• Electrons present in the outermost shell are called valence electrons.
• Valence electrons are the electrons mainly involved in chemical bonding.
• Inner-shell electrons are generally not involved in chemical combination.

Example

Na = 2, 8, 1

• Inner electrons = 10
• Valence electrons = 1

Cl = 2, 8, 7

• Inner electrons = 10
• Valence electrons = 7

⭐ NEET Shortcut

For main-group elements, outer-shell electrons are usually the valence electrons.

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7. Lewis symbols

• G. N. Lewis introduced a simple way to represent valence electrons.
• These representations are called Lewis symbols.
• The element symbol is written in the centre.
• Dots are placed around the symbol.
• The number of dots = number of valence electrons.

Example

If an atom has:

• 1 valence electron → 1 dot
• 2 valence electrons → 2 dots
• 3 valence electrons → 3 dots
• 4 valence electrons → 4 dots
• 5 valence electrons → 5 dots
• 6 valence electrons → 6 dots
• 7 valence electrons → 7 dots
• 8 valence electrons → 8 dots

⭐ Very important

Number of dots in a Lewis symbol = number of valence electrons.

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8. Significance of Lewis symbols

Lewis symbols help us understand:

1. Number of valence electrons

• Simply count the dots.

2. Valency

For many main-group elements, the common valency can be understood as:

Valency = number of valence electrons

or

Valency = 8 − number of valence electrons

Use the value that gives the smaller number for the common valency.

Examples

Na: 1 valence electron
→ valency = 1

Mg: 2 valence electrons
→ valency = 2

Al: 3 valence electrons
→ valency = 3

C: 4 valence electrons
→ valency = 4

N: 5 valence electrons
→ valency = 8 − 5 = 3

O: 6 valence electrons
→ valency = 8 − 6 = 2

F: 7 valence electrons
→ valency = 8 − 7 = 1

Ne: 8 valence electrons
→ valency = 0

⭐ Easy pattern

Valence electrons	Common valency
1	1
2	2
3	3
4	4
5	3
6	2
7	1
8	0

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9. Kössel's approach

Kössel focused particularly on ionic bond formation.

He highlighted four important facts.

Point 1: Position of metals and non-metals

• Highly electropositive alkali metals and highly electronegative halogens are separated in the periodic table by the noble gases.
• Alkali metals tend to lose electrons.
• Halogens tend to gain electrons.

Point 2: Formation of ions

• An alkali metal forms a positive ion by losing electron(s).
• A halogen forms a negative ion by gaining electron(s).

Example:

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

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Point 3: Noble gas configuration

• The ions formed after electron transfer achieve a stable electronic configuration.
• This configuration is generally similar to that of a noble gas.

Noble gas outer configuration

For most noble gases:

ns² np⁶

This means:

2 + 6 = 8 electrons → octet

Exception: Helium

• Helium has only two electrons.
• Its stable configuration is:

1s²

• Therefore, helium follows a duplet, not an octet.

⭐ NEET trap

Helium = duplet (2 electrons)
Other noble gases = generally octet (8 electrons)

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10. Electrostatic attraction

• After electron transfer, positive and negative ions are produced.
• Opposite charges attract each other.
• This attraction is called electrostatic attraction.
• It holds the ions together in an ionic compound.

For NaCl:

Na⁺ ↔ Cl⁻

The attraction between them produces the ionic bonding.

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11. Formation of NaCl — step by step

Step 1: Sodium loses an electron

Electronic configuration:

Na = [Ne] 3s¹

After losing one electron:

Na → Na⁺ + e⁻

Therefore:

Na⁺ = [Ne]

Sodium ion has achieved the stable configuration of neon.

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Step 2: Chlorine gains an electron

Electronic configuration:

Cl = [Ne] 3s² 3p⁵

After gaining one electron:

Cl + e⁻ → Cl⁻

Therefore:

Cl⁻ = [Ne] 3s² 3p⁶ = [Ar]

Chloride ion has achieved the stable configuration of argon.

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Step 3: Ions attract

Na⁺ + Cl⁻ → NaCl

Therefore:

Electron transfer + electrostatic attraction = ionic bonding

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12. Formation of CaF₂ — step by step

Step 1: Calcium loses two electrons

Calcium:

Ca = [Ar] 4s²

It loses two electrons:

Ca → Ca²⁺ + 2e⁻

Therefore:

Ca²⁺ = [Ar]

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Step 2: Fluorine gains one electron

Fluorine:

F = [He] 2s² 2p⁵

It needs 1 electron to complete its octet.

F + e⁻ → F⁻

Therefore:

F⁻ = [He] 2s² 2p⁶ = [Ne]

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Step 3: Charge balance

• Ca loses 2 electrons.
• Each F atom gains 1 electron.
• Therefore, one Ca atom needs two F atoms.

So:

Ca²⁺ + 2F⁻ → CaF₂

⭐ NEET shortcut

For an ionic compound, total positive charge = total negative charge.

For CaF₂:

(+2) + 2(−1) = 0

Therefore, the formula is CaF₂.

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13. Electrovalent bond

• The bond formed due to electrostatic attraction between oppositely charged ions is called an electrovalent bond.
• Today, the more commonly used term is ionic bond.

Electrovalence

• Electrovalence is related to the number of unit charges present on an ion.
• Calcium forms:

Ca²⁺

So calcium has a positive electrovalence of 2.

• Chlorine forms:

Cl⁻

So chlorine has a negative electrovalence of 1.

⭐ Remember

Ca²⁺ → electrovalence = +2

Cl⁻ → electrovalence = −1

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14. Importance of Kössel's postulates

Kössel's ideas provided the foundation for understanding:

• Formation of ions by electron transfer
• Formation of ionic compounds
• Stability of ions through noble-gas-like configurations
• Electrostatic attraction between oppositely charged ions
• Ionic crystalline structures

But there is a limitation

• Not all compounds can be explained using only electron transfer.
• Many compounds involve sharing of electrons.
• Therefore, the Kössel approach alone cannot explain all types of chemical bonding.
• Lewis's concept of electron sharing helps explain covalent compounds.

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🔥 NEET Quick Revision

Learn these 10 points:

1. Kössel and Lewis → 1916
2. Main idea → atoms seek stable electronic configuration.
3. Valence electrons → outermost-shell electrons.
4. Lewis symbol → element symbol + dots representing valence electrons.
5. Octet → 8 valence-shell electrons.
6. Helium → duplet (2 electrons).
7. Electron loss → cation (+).
8. Electron gain → anion (−).
9. Electron transfer + electrostatic attraction → ionic bond.
10. Electron sharing → covalent bond.

🧠 One-line memory trick

“Lose → Positive, Gain → Negative, Share → Covalent, Opposite charges → Ionic.”

Important examples

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na⁺ + Cl⁻ → NaCl

Ca → Ca²⁺ + 2e⁻

2F + 2e⁻ → 2F⁻

Ca²⁺ + 2F⁻ → CaF₂

Noble gas stability → Octet (except He → Duplet) 

KÖSSEL–LEWIS APPROACH TO CHEMICAL BONDING

Class 11 CBSE — Complete Question Bank with Answers

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A. MCQs — Multiple Choice Questions

1. Kössel and Lewis independently proposed their explanation of chemical bonding in:

A. 1905
B. 1916
C. 1920
D. 1932

Answer: B. 1916

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2. The main basis of the Kössel–Lewis approach is:

A. Atomic mass
B. Nuclear stability
C. Noble gas electronic configuration
D. Atomic size

Answer: C. Noble gas electronic configuration

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3. Electrons present in the outermost shell of an atom are called:

A. Core electrons
B. Valence electrons
C. Free electrons
D. Nuclear electrons

Answer: B. Valence electrons

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4. The number of dots in a Lewis symbol represents:

A. Number of neutrons
B. Number of protons
C. Number of valence electrons
D. Atomic number

Answer: C. Number of valence electrons

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5. The maximum number of electrons in the outer shell according to the octet concept is:

A. 2
B. 4
C. 6
D. 8

Answer: D. 8

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6. Which noble gas has a duplet configuration?

A. Ne
B. Ar
C. He
D. Kr

Answer: C. He

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7. The stable outer-shell configuration of most noble gases is:

A. ns¹
B. ns²
C. ns²np⁶
D. np⁶

Answer: C. ns²np⁶

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8. Sodium forms Na⁺ by:

A. Gaining one electron
B. Losing one electron
C. Sharing two electrons
D. Gaining two electrons

Answer: B. Losing one electron

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9. Chlorine forms Cl⁻ by:

A. Losing one electron
B. Losing two electrons
C. Gaining one electron
D. Sharing one electron

Answer: C. Gaining one electron

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10. The bond formed by electrostatic attraction between oppositely charged ions is called:

A. Covalent bond
B. Hydrogen bond
C. Ionic/electrovalent bond
D. Metallic bond

Answer: C. Ionic/electrovalent bond

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11. In NaCl, sodium has the electronic configuration:

A. [Ne]
B. [Ne]3s¹
C. [Ar]
D. [He]

Answer: A. [Ne]

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12. The electronic configuration of Cl⁻ is:

A. [Ne]3s²3p⁵
B. [Ne]3s²3p⁶
C. [Ar]4s¹
D. [He]2s²2p⁵

Answer: B. [Ne]3s²3p⁶

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13. Calcium forms Ca²⁺ by:

A. Gaining two electrons
B. Losing one electron
C. Losing two electrons
D. Sharing two electrons

Answer: C. Losing two electrons

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14. The correct formula formed by Ca²⁺ and F⁻ is:

A. CaF
B. Ca₂F
C. CaF₂
D. Ca₂F₂

Answer: C. CaF₂

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15. In the formation of CaF₂, one calcium atom transfers:

A. 1 electron
B. 2 electrons
C. 3 electrons
D. 4 electrons

Answer: B. 2 electrons

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16. In Cl₂, the two chlorine atoms are bonded by:

A. Transfer of electrons
B. Sharing of an electron pair
C. Transfer of protons
D. Sharing of protons

Answer: B. Sharing of an electron pair

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17. Which of the following represents a covalent molecule?

A. NaCl
B. CaF₂
C. Cl₂
D. Na⁺

Answer: C. Cl₂

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18. The inner-shell electrons are generally:

A. Directly involved in chemical bonding
B. Not involved in chemical bonding
C. Always transferred
D. Always shared

Answer: B. Not involved in chemical bonding

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19. The positive ion formed by loss of electrons is called:

A. Anion
B. Cation
C. Radical
D. Molecule

Answer: B. Cation

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20. The negative ion formed by gain of electrons is called:

A. Cation
B. Anion
C. Proton
D. Kernel

Answer: B. Anion

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B. Very Short Answer Questions

1. Who proposed the Kössel–Lewis approach?

Answer: Kössel and G. N. Lewis independently proposed the approach in 1916.

2. What are valence electrons?

Answer: Electrons present in the outermost shell of an atom are called valence electrons.

3. What does a Lewis symbol represent?

Answer: It represents an element's valence electrons using dots around its chemical symbol.

4. What is an octet?

Answer: An octet is a stable arrangement containing eight electrons in the valence shell.

5. What is a duplet?

Answer: A duplet is a stable arrangement containing two electrons in the outer shell, as in helium.

6. What happens when sodium loses one electron?

Answer: Sodium forms Na⁺.

7. What happens when chlorine gains one electron?

Answer: Chlorine forms Cl⁻.

8. What is an electrovalent bond?

Answer: It is the bond formed due to electrostatic attraction between oppositely charged ions.

9. What is electrovalence?

Answer: Electrovalence is the number of unit charges carried by an ion.

10. How many valence electrons does chlorine have?

Answer: Chlorine has seven valence electrons.

11. How many valence electrons does sodium have?

Answer: Sodium has one valence electron.

12. Which electrons generally participate in chemical bonding?

Answer: Valence electrons.

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C. Short Answer Questions

1. What is the basic idea of the Kössel–Lewis approach?

Answer:

• Atoms tend to attain a stable electronic configuration.
• This configuration is generally similar to that of noble gases.
• Most atoms achieve an octet of valence electrons.
• They may achieve this by losing, gaining or sharing electrons.

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2. Explain the term valence electrons.

Answer:

• Electrons present in the outermost shell are called valence electrons.
• They are mainly responsible for chemical combination.
• They determine many chemical properties and the valency of an element.
• Inner-shell electrons are generally not involved in chemical bonding.

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3. What are Lewis symbols?

Answer:

• G. N. Lewis introduced a simple notation for representing valence electrons.
• The chemical symbol of an element is written in the centre.
• Dots are placed around the symbol.
• The number of dots represents the number of valence electrons.

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4. Explain the formation of NaCl according to Kössel's concept.

Answer:

Sodium has configuration:

Na = [Ne]3s¹

It loses one electron:

Na → Na⁺ + e⁻

Thus:

Na⁺ = [Ne]

Chlorine has configuration:

Cl = [Ne]3s²3p⁵

It gains one electron:

Cl + e⁻ → Cl⁻

Thus:

Cl⁻ = [Ne]3s²3p⁶ = [Ar]

The Na⁺ and Cl⁻ ions attract each other electrostatically:

Na⁺ + Cl⁻ → NaCl

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5. Why do atoms form chemical bonds according to the Kössel–Lewis approach?

Answer: Atoms form chemical bonds to attain greater stability. They tend to achieve a stable electronic configuration similar to noble gases, usually an octet of valence electrons.

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6. Differentiate between cation and anion.

Answer:

Cation	Anion
Positively charged ion	Negatively charged ion
Formed by loss of electrons	Formed by gain of electrons
Example: Na⁺	Example: Cl⁻
Number of electrons decreases	Number of electrons increases

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D. Long Answer Questions

1. Explain the Kössel–Lewis approach to chemical bonding.

Answer:

The Kössel–Lewis approach was independently proposed by Kössel and Lewis in 1916.

The important points are:

1. Noble gases have particularly stable electronic configurations.
2. Most noble gases have eight electrons in their valence shell.
3. Atoms tend to attain a similar stable configuration.
4. Atoms can achieve stability by losing, gaining or sharing electrons.
5. Loss of electrons produces positively charged ions.
6. Gain of electrons produces negatively charged ions.
7. Oppositely charged ions attract each other electrostatically and form ionic compounds.
8. Atoms can also share electron pairs to form covalent bonds.
9. Valence electrons are the electrons mainly involved in chemical bonding.
10. The Lewis symbol represents valence electrons as dots around the element symbol.

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2. Explain the formation of CaF₂ according to Kössel's approach.

Answer:

Calcium has the electronic configuration:

Ca = [Ar]4s²

It loses two electrons:

Ca → Ca²⁺ + 2e⁻

Thus:

Ca²⁺ = [Ar]

Fluorine has the configuration:

F = [He]2s²2p⁵

Each fluorine atom requires one electron to complete its octet.

Therefore:

F + e⁻ → F⁻

Two fluorine atoms are required to accept the two electrons lost by one calcium atom.

Therefore:

Ca²⁺ + 2F⁻ → CaF₂

The oppositely charged ions are held together by electrostatic attraction.

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3. Explain Lewis symbols and their significance.

Answer:

Lewis symbols were introduced by G. N. Lewis to represent valence electrons.

• The element symbol represents the atom.
• Dots around the symbol represent valence electrons.
• The number of dots equals the number of valence electrons.
• Lewis symbols help us understand the tendency of atoms to lose, gain or share electrons.
• They also help in understanding the common valency of main-group elements.

For example:

• Na has 1 valence electron → one dot.
• Mg has 2 valence electrons → two dots.
• Cl has 7 valence electrons → seven dots.
• Ne has 8 valence electrons → eight dots.

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E. Assertion–Reason Questions

Instructions:

Choose the correct option:

A. Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
B. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
C. Assertion is true, but Reason is false.
D. Assertion is false, but Reason is true.

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1.

Assertion: Noble gases are chemically less reactive.

Reason: They generally have stable valence-shell configurations.

Answer: A

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2.

Assertion: Sodium forms Na⁺ by gaining one electron.

Reason: Sodium has one valence electron.

Answer: D

Explanation: Sodium forms Na⁺ by losing one electron.

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3.

Assertion: Chlorine forms Cl⁻ by gaining one electron.

Reason: Chlorine has seven valence electrons.

Answer: A

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4.

Assertion: NaCl contains oppositely charged ions.

Reason: Sodium loses an electron and chlorine gains an electron.

Answer: A

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5.

Assertion: Helium follows the octet rule.

Reason: Helium has two electrons in its only shell.

Answer: D

Explanation: Helium follows the duplet rule.

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6.

Assertion: CaF₂ contains two fluoride ions for every calcium ion.

Reason: Calcium forms Ca²⁺ and each fluorine forms F⁻.

Answer: A

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7.

Assertion: Lewis symbols contain dots around the chemical symbol.

Reason: The dots represent valence electrons.

Answer: A

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8.

Assertion: Inner-shell electrons are generally involved in chemical bonding.

Reason: Inner-shell electrons are strongly held by the nucleus.

Answer: D

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F. Fill in the Blanks

1.

Kössel and Lewis proposed their explanation of chemical bonding in the year ________.

Answer: 1916

2.

Electrons present in the outermost shell are called ________ electrons.

Answer: valence

3.

G. N. Lewis introduced ________ symbols.

Answer: Lewis

4.

The stable configuration of most noble gases contains ________ electrons in the valence shell.

Answer: eight

5.

Helium has a stable ________ configuration.

Answer: duplet

6.

Sodium loses one electron to form ________.

Answer: Na⁺

7.

Chlorine gains one electron to form ________.

Answer: Cl⁻

8.

An ion carrying a positive charge is called a ________.

Answer: cation

9.

An ion carrying a negative charge is called an ________.

Answer: anion

10.

The bond formed by electrostatic attraction between oppositely charged ions is called an ________ bond.

Answer: ionic/electrovalent

11.

The electronic configuration of noble gases except helium is generally ________.

Answer: ns²np⁶

12.

Calcium loses ________ electrons to form Ca²⁺.

Answer: two

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G. Statement-Based Questions

1. Consider the following statements:

1. Valence electrons are present in the outermost shell.
2. Inner-shell electrons generally participate in bonding.
3. Lewis symbols represent valence electrons by dots.
4. Noble gases generally have stable valence-shell configurations.

Which statements are correct?

A. 1 and 2 only
B. 1, 3 and 4 only
C. 2 and 3 only
D. 1, 2, 3 and 4

Answer: B

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2. Consider the following statements about NaCl:

1. Na loses one electron.
2. Cl gains one electron.
3. Na becomes Na⁻.
4. Cl becomes Cl⁻.

Correct statements are:

A. 1 and 2 only
B. 2 and 3 only
C. 1, 2 and 4 only
D. All four

Answer: C

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3. Consider the following statements:

1. Calcium forms Ca²⁺.
2. Fluorine forms F⁻.
3. Two fluoride ions are needed for one Ca²⁺ ion.
4. The formula is CaF₂.

The correct answer is:

A. 1 only
B. 1 and 2 only
C. 1, 2 and 3 only
D. 1, 2, 3 and 4

Answer: D

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4. Which statements are correct regarding Lewis symbols?

1. Dots represent valence electrons.
2. The chemical symbol is placed at the centre.
3. They represent the nucleus in detail.
4. They help in understanding chemical bonding.

A. 1, 2 and 4
B. 1 and 3
C. 2 and 3
D. All

Answer: A

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H. Match the Columns

Question 1

Column I	Column II
A. Na	1. F⁻
B. Cl	2. Na⁺
C. Ca	3. Ca²⁺
D. F	4. Cl⁻

Answer:
A → 2
B → 4
C → 3
D → 1

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Question 2

Column I	Column II
A. Loss of electron	1. Anion
B. Gain of electron	2. Cation
C. Eight valence electrons	3. Octet
D. Two valence electrons in He	4. Duplet

Answer:
A → 2
B → 1
C → 3
D → 4

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Question 3

Column I	Column II
A. Na⁺	1. [Ar]
B. Cl⁻	2. [Ne]
C. Ca²⁺	3. [Ne]
D. F⁻	4. [Ne]

Answer:
A → 2
B → 1
C → 1
D → 1

Note: Na⁺, Ca²⁺ and F⁻ have [Ne], [Ar], and [Ne] respectively; hence the properly arranged matching is:

• Na⁺ → [Ne]
• Cl⁻ → [Ar]
• Ca²⁺ → [Ar]
• F⁻ → [Ne]

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I. Case Study-Based Questions

Case Study 1: Formation of Sodium Chloride

Sodium has electronic configuration [Ne]3s¹, whereas chlorine has electronic configuration [Ne]3s²3p⁵. Sodium tends to lose one electron and chlorine tends to gain one electron. The resulting Na⁺ and Cl⁻ ions attain stable noble-gas configurations and are held together by electrostatic attraction.

Questions:

1. How many valence electrons does sodium have?

A. 1
B. 2
C. 7
D. 8

Answer: A. 1

2. What ion is formed by chlorine?

A. Cl⁺
B. Cl²⁺
C. Cl⁻
D. Cl²⁻

Answer: C. Cl⁻

3. What type of bond is present in NaCl?

Answer: Ionic/electrovalent bond.

4. What is the configuration of Na⁺?

Answer: [Ne]

5. Why are Na⁺ and Cl⁻ attracted to each other?

Answer: They have opposite charges, so they experience electrostatic attraction.

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Case Study 2: Formation of Calcium Fluoride

Calcium has the electronic configuration [Ar]4s². It loses two electrons to attain the stable [Ar] configuration. Fluorine has seven valence electrons and each fluorine atom gains one electron. Therefore, two fluorine atoms combine with one calcium atom to form CaF₂.

Questions:

1. What ion is formed by calcium?

Answer: Ca²⁺

2. How many electrons does calcium lose?

Answer: Two electrons.

3. How many electrons does one fluorine atom gain?

Answer: One electron.

4. Why are two fluorine atoms required?

Answer: One Ca atom loses two electrons, while each F atom gains only one electron. Therefore, two F atoms are required.

5. Write the formation equation.

Answer:

Ca → Ca²⁺ + 2e⁻

2F + 2e⁻ → 2F⁻

Ca²⁺ + 2F⁻ → CaF₂

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J. HOTS / Application-Based Questions

1. An atom has 7 valence electrons. What will it most likely do to achieve a stable octet?

Answer: It will most likely gain one electron and form a −1 ion.

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2. An atom has one valence electron. What type of ion will it generally form?

Answer: It will generally lose one electron and form a +1 cation.

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3. Why does CaF₂ contain one Ca²⁺ ion and two F⁻ ions?

Answer: Because the total charge must be zero:

(+2) + 2(−1) = 0

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4. An element has five valence electrons. What is its common valency?

Answer: 3, because it needs three electrons to complete its octet.

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5. Why is the formula of calcium fluoride not CaF?

Answer: Calcium forms Ca²⁺, whereas fluorine forms F⁻. Two F⁻ ions are required to balance one Ca²⁺ ion. Therefore, the formula is CaF₂.

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K. One-Mark Revision Questions

1. Who introduced Lewis symbols?
   Ans: G. N. Lewis.

2. What do dots in a Lewis symbol represent?
   Ans: Valence electrons.

3. What is the stable configuration of helium?
   Ans: Duplet.

4. What is the stable configuration of most noble gases?
   Ans: Octet.

5. What is formed when an atom loses electrons?
   Ans: Cation.

6. What is formed when an atom gains electrons?
   Ans: Anion.

7. What type of force holds Na⁺ and Cl⁻ together?
   Ans: Electrostatic attraction.

8. What is the formula of calcium fluoride?
   Ans: CaF₂.

9. How many valence electrons are present in Cl?
   Ans: 7.

10. How many electrons does Ca lose?
    Ans: 2.

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L. Important 3–5 Mark Questions for CBSE

Q1. Explain the significance of Lewis symbols.

Answer points:

• Introduced by G. N. Lewis.
• Represent valence electrons using dots.
• Number of dots = number of valence electrons.
• Help determine the tendency of an atom to lose, gain or share electrons.
• Help understand common valency.
• Useful for explaining chemical bonding.

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Q2. Describe the formation of NaCl using electronic configurations.

Answer points:

Na = [Ne]3s¹

Na loses one electron:

Na → Na⁺ + e⁻

Na⁺ = [Ne]

Cl = [Ne]3s²3p⁵

Cl gains one electron:

Cl + e⁻ → Cl⁻

Cl⁻ = [Ar]

Then:

Na⁺ + Cl⁻ → NaCl

The ions are held together by electrostatic attraction.

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Q3. Explain Kössel's postulates related to ionic bond formation.

Answer points:

1. Alkali metals are highly electropositive.
2. Halogens are highly electronegative.
3. Alkali metals tend to lose electrons.
4. Halogens tend to gain electrons.
5. Electron transfer produces positive and negative ions.
6. The ions attain noble-gas-like stable configurations.
7. Oppositely charged ions are stabilized by electrostatic attraction.
8. The resulting bond is called an ionic/electrovalent bond.

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M. Final NEET + CBSE Concept Map

Atom

↓

Valence electrons

↓

Atom seeks stable configuration

↓

Lose / Gain / Share electrons

↓

Lose electrons → Cation (+)

Gain electrons → Anion (−)

Share electrons → Covalent bond

↓

Cation + Anion

↓

Electrostatic attraction

↓

Ionic / Electrovalent bond

⭐ Must Remember

Na → Na⁺ + e⁻

Cl + e⁻ → Cl⁻

Na⁺ + Cl⁻ → NaCl

Ca → Ca²⁺ + 2e⁻

2F + 2e⁻ → 2F⁻

Ca²⁺ + 2F⁻ → CaF₂

Octet = 8 electrons

Duplet = 2 electrons

Lewis dots = valence electrons

Loss → positive

Gain → negative

Sharing → covalent

Opposite ions → electrostatic attraction → ionic bond

internal-link structure

• Class 11 Chemistry Notes → Link to your main Class 11 Chemistry notes page.
• Chemical Bonding and Molecular Structure Notes → Link to the detailed chapter notes.
• Class 11 Chemistry MCQs → Link to your Class 11 MCQ collection.
• Periodic Classification of Elements → Link to the previous/relevant chapter.
• Structure of Atom Class 11 → Link because electronic configuration is important before Chemical Bonding.
• Thermodynamics Class 11 Chemistry → Link where energy and stability are discussed.
• Equilibrium Class 11 Chemistry → Link to the relevant Class 11 chapter. 
• Organic Chemistry – Some Basic Principles and Techniques → Link for students progressing to organic chemistry.
• CBSE Class 11 Chemistry Question Bank → Link to the complete question-bank category.
• CBSE Class 11 Chemistry Important Questions → Link to your important-question collection.
• CBSE Class 11 Chemistry Case Study Questions → Link to case-study practice.
• CBSE Class 11 Chemistry Assertion and Reason Questions → Link to assertion-reason practice.