Chemistry Class 9th Chapter 4 notes

Important questions covered:

• State octet and duplet rule?
• What is a chemical bond?
• Why do atoms form a chemical bond?
• Explain the two concepts, which explain the chemical bonding?
• What is an Ionic bond? Explain with an example.
• Write a note on the Covalent bond?
• How does Al form cation?
• How does oxygen form an anion?
• Write down the importance of noble gas electronic configuration.
• What is the main distinction between ionic and covalent bonds? Explain your answer with suitable examples.
• Classify the following substances as covalent or Ionic.

1. NaCl
2. CO₂
3. MgCl₂
4. H₂O
5. C₆H₆
6. HCl

• In what way do atoms form ions?
• An ionic bond is formed between which two types of elements?
• State the formula of ions formed by the following atoms?
• Ca ²⁰
• F⁹
• Li³
• O⁸
• Draw diagrams to show the ionic bond in MgO and CaCl2?
• Define valence electrons?
• Define valence shell?
• What is the importance of glues and adhesives in our society?

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Important Q/A

State octet and duplet rule?

Octet Rule:

Octet means “eight”

The tendency of an atom to attain eight electrons in its valence shell and becomes stable is known as the octet rule.

Duplet Rule:

Duplet means “two”

The tendency of an atom to attain two electrons in its valence shell and becomes stable is known as the duplet rule.

Watch Octet and Duplet Rule

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What is a chemical bond?

OR

Identify and define the four major types of chemical bonding?

Answer:

Chemical Bond

The attractive force which holds the atom together to form a chemical compound or molecule is called a chemical bond.

Watch Chemical bonding in Hindi / Urdu

EXPLANATION:

1. The chemical reactivity of free elements other than an inert gas element is due to incomplete octet in their outermost shell.
2. Therefore, to attain stability and to complete the outermost shell with electrons, atoms combine chemically to form chemical bonds.

EXAMPLE

1. Molecules of the same elements consist of the same kind of atoms

O2

O3

1. While molecules of the compounds are made up of different kinds of atoms.

H20

CO2

Types of Chemical Bonds:

Chemical bonds are of the following types

1. Ionic bond
2. Covalent bond
3. Co-ordinate covalent bond
4. Metallic bond

Ionic Bond:

The chemical bond which is formed by the complete transfer of electrons from one atom to another atom is called an Ionic bond.

EXAMPLE

NaCl

CaCl2

Watch Ionic bond in Hindi / Urdu

Covalent Bond:

The chemical bond formed by the mutual sharing of electrons between atoms is called a Covalent bond.

EXAMPLE:

H2O

O2

Watch recommended Tutorials:

Single,double and Triple Covalent Bond formation

Difference between Polar and non-polar bond

Is H2O polar or non-polar

Why HF polar

Co-ordinate covalent bond:

The covalent bond in which shared pair of an electron is donated by one of the two bonded atoms is called Co-ordinate covalent bond.

EXAMPLE:

NH4 (Ammonium Ion)

H3O

Watch recommended Tutorial:

Co-ordinate bond

Metallic bond:

The positively charged ions are held together by mobile electrons are called Metallic bonds.

EXAMPLE:

All metals have metallic bonds.

Watch How metallic bonds are formed?

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Why do atoms form a chemical bond?

OR

Why most atoms are chemically bonded to other atoms in nature?

ANSWER:

Chemical bond:

The attractive force which holds the atoms together to form a compound or molecule is called a Chemical bond.

REASONS:

1. Every system in the universe tends to lower its energy, to attain stability.
2. Water flows from a higher level to a lower level.
3. Similarly, electricity flows from higher potential to lower potential and heat flows from a hot body to a cold body.
4. This happens because both water and electricity are trying to decrease their energy.
5. Atoms in the same way tend to decrease their energy.
6. The energy of isolated hydrogen atoms is higher than the two bonded hydrogen atoms.
7. That is, the combination of atoms gives a stable molecule through the emission of energy.
8. They can decrease the energy by combining with other atoms and forming a chemical bond.

EXAMPLE:

H + H 🡪H—H🡪H₂

Cl +Cl 🡪Cl—Cl🡪Cl₂

CONCLUSION:

It is concluded from the above-mentioned reason that atoms form a chemical bond.

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Explain the two concepts, which explain the chemical bonding?

ANSWER:

Chemical bonding is explained with the help of two important concepts.

1. The Valence Concept
2. The Orbital Concept

The Valence Concept (Electric theory of valence)

INTRODUCTION:

In 1916, G.N Lewis and W. Kossel presented the electronic theory of valence.

STATEMENT:

It states that “In a chemical bond formation, atoms take part by losing, gain or sharing of electrons, to attain the inert or noble gas electronic configuration”

EXPLANATION:

• When atoms have two or eight electrons in their outermost shell, they are stable.
• The electron theory of valence can be explained named as Octet or Duplet’s theory of valance.

1. Octet Theory Of Valence / Rule Of Eight:

“The tendency of atoms to attain eight electrons in the outermost shell to attain stability.”

EXAMPLE:

1. Oxygen (O) has six electrons in its valence shell.
2. It shares or gains two electrons in its outermost shell and attains stability by completing its outermost shell with eight electrons.

This is also named “The Octet rule or the rule of eight”.

Duplet Rule/Rule Of Two:

“The tendency of atoms to attain two valance electrons to attain stability.”

EXAMPLE:

• Helium (He) has two electrons in its valence shell and is stable.
• The elements of group 8 of the periodic table, such as helium, neon, and argon are known as Noble Gases.
• They are also known as Inert Gases because they are very stable and rarely take part in chemical reactions to form compounds.
• Their stability comes from their filled outermost shell.
• Except for Helium which has two electrons in its outermost shell, all other noble gasses have their outermost shell filled with eight electrons.
• A shell with eight electrons is called an Octet and is very stable.
• Thus, when atoms take part in chemical reactions, they tend to combine in ways to complete eight electrons in their outermost shell, to attain the electronic configuration of the noble gasses. (except Helium)

The Orbital Concept:

1. This concept is based on the combination of atomic orbital to produce molecular orbital.
2. The atomic orbital has one electron.
3. These orbitals when coming close to one another, overlap each other.
4. This overlapping is either Endwise or Sidewise.
5. Endwise overlapping produces Sigma Bond.
6. Sidewise overlapping produces Pi-Bond.

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What is an Ionic bond? Explain with an example.

ANSWER:

Ionic Bond:

“The chemical bond which is formed due to complete transfer of electrons from one atom to another atom is called Ionic bond or electrovalent bond”.

Condition:

Following are the conditions that are necessary for the formation of an Ionic bond.

• One element should be metal while other elements should be non-metal.
• The electronegativity difference between two bonded atoms should be 1.7 or above.
• The ionization energy of metal should be low.
• The electronegativity of non-metal should be high.

EXPLANATION:

1. The transfer of electrons between atoms completes the octet or duplet.
2. This type of bonding is always between metal and non-metal.
3. Metals always lose electrons to form Cations and non-metals gain electrons to form Anions.
4. In ionic bond formation, one atom loses an electron, and the other gain it.
5. The atom that losses the electron acquires a positive charge and the other atom, which gains an electron becomes a negatively charged particle.
6. Due to opposite charges, the electrostatic force of attraction is set up between them.
7. This force holds these ions together.
8. This force of attraction is referred to as an ionic bond.
9. An atom having low ionization energy will lose one or more electrons to form a cation.
10. An atom having high electronegativity will gain one or more electrons to form an anion.

EXAMPLE:

1. Formation Of Sodium Chloride (NaCl):

1. Sodium (Na) and chloride (Cl) combine to form sodium chloride (NaCl).
2. The atomic number of sodium is 11.
3. The electronic configuration of sodium is:

Based On Shell:

K = 2

L = 8

M = 1

Based On Sub-Shell:

1s², 2s², 2p⁶, 3s¹

1. Sodium belongs to group 1 A
2. Chlorine belongs to group 7 A
3. The electronegativity of sodium is 0.9 and chlorine is 3.0
4. The electronegativity difference between sodium and chlorine is 2.1, hence, an ionic bond is formed.
5. Sodium has one electron in its outermost shell. Sodium loses one electron to form sodium ion (Na⁺) and attain the electronic configuration of neon (Ne).

Energy

Na_(s) 🡪 Na⁺ + 1e^–

1. The atomic number of chlorine is 7.
2. The electronic configuration of chlorine is:

Based On Shell:

K=2

L=8

M=7

Based On Sub-Shell:

1s², 2s², 2p⁶, 3s², 3p⁵

1. Chlorine has 7 electrons in its outermost shell.
2. Cl gains one electron to form a chloride ion (Cl^–) and to attain the electronic configuration of Argon (Ar).

Cl + 1e^–🡪 Cl^–

1. The electrostatic force f attraction is established between sodium and chlorine
2. Therefore, a crystal of NaCl is formed.

Na⁺+ Cl^– 🡪Na-Cl

Ionic bond

1. Formation Of Calcium Chloride (CaCl2):
2. Ca belongs to group II A
3. Cl belongs to group VII A
4. The electronegativity of calcium is 1.0 while chlorine is 3.0
5. The electronegativity difference between calcium and chlorine is 2.0
6. Thus, an ionic bond is formed
7. Calcium has 2 electrons in its valence shell
8. It tends to lose electrons to attain the inert gas electronic configuration of Neon (Ne)
9. Thus, the calcium ion (Ca⁺) is formed

Ca ——-🡪 Ca⁺² + 2^e-

1. Chlorine needs one electron to complete its valence shell
2. It tends to gain one electron to attain the inert gas electronic configuration of Argon (Ar)
3. So, it forms chlorine ion

2Cl +2e^– 🡪Cl^–

1. The electrostatic force of attraction appears between Ca⁺² and two chloride ions (Cl^– )
2. Thus, white colored crystalline CaCl2 is formed

Ca⁺² + 2Cl^– 🡪CaCl₂

1. Formation Of AlCl3:
2. Al belongs to group III A
3. Cl belongs to group VII A
4. Al has three electrons in its valence shell while Cl has 7 electrons in its valence shell
5. Al loses three electrons to form Aluminum ion (Al⁺³)

Al 🡪 3e^–+ Al ⁺³

1. Each Cl atom gains one electron to form a chloride ion (Cl^–)

3Cl + 3e^– 🡪3Cl^–

1. The electrostatic force that appears between (Al⁺³) and (Cl^–) ions
2. Thus, ALCl₃shows ionic character

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Write a note on the Covalent bond?

ANSWER:

• COVALENT BOND:

INTRODUCTION:

G.N Lewis proposed that atoms could attain stable electronic configuration by sharing electrons between them

DEFINITION:

The bond formed by the mutual sharing of electrons is called a covalent bond.

REPRESENTATION:

The shared pair of electrons are represented by a dash (-) between the two bonded atoms.

CONDITION:

1. Both the elements should be non- metals
2. The electronegativity difference between two bonded atoms should be less than 1.7

EXAMPLE:

• Hydrogen molecule (H₂)
• Oxygen molecule (O₂)
• Hydrochloric acid (HCl)
• Hydrogen bromide (HBr)

Types Of Covalent Bond:

The covalent bond is further divided into three types:

1. Single covalent bond
2. Double covalent bond
3. Triple covalent bond

Single Covalent Bond:

“The bond in which the two atoms share one electron each to form a pair of electrons is called the single covalent bond.”

REPRESENTATION:

A single straight line (-) shows the single covalent bond.

EXAMPLE:

H₂

Cl₂

F₂

Example of Cl₂:

• A chlorine molecule is formed from two chlorine atoms.
• The chlorine atom’s electronic configuration is (2,8,7)
• A chlorine atom has 7 electrons in its valence shell
• The two chlorine atoms mutually share one atom to form a chlorine molecule (Cl₂)
• Therefore, both chlorine atoms attain the electronic configuration of (Ar) and complete their octet

Example of CH₄:

• Carbon has four electrons in its outermost shell and needs more electrons to attain the nearest noble gas configuration
• Therefore, four atoms of hydrogen mutually share one electron each with a carbon atom to form a molecule of methane (CH₄)

Double Covalent Bond:

“The bond in which two atoms share two electrons each to form two pairs of electrons is called a double covalent bond.

REPRESENTATION:

A double straight line shows such a covalent bond

EXAMPLE:

O₂

C₂H₄

CO₂

Example of Oxygen (O₂):

• An oxygen molecule is formed from two oxygen atoms
• The oxygen atom electronic configuration is (2,6)
• An oxygen atom has six electrons in its valence shell and it shares two electrons with another oxygen atom to form an oxygen molecule (O₂)
• In this way, both oxygen atoms attain inert gas neon (Ne) electronic configuration and complete their octet.

O + O 🡪 O₂

Example of Carbon Dioxide

Similarly in carbon dioxide, the carbon atom shares four electrons with two oxygen atoms and forms two double covalent bonds

O + C + O 🡪 CO₂

Triple Covalent Bond

The bond in which two atoms share three electrons each, to form three pairs of an electron is called a triple covalent bond

Representation

It is represented by triple straight lines

Examples

1. N₂ ii) H₂

Examples of Nitrogen (N₂)

The nitrogen molecule is formed from two nitrogen atoms.

The nitrogen atom’s electronic configuration is (2,5)

A nitrogen atom has five electrons in its valance shell and it shares three electrons with another nitrogen atom to form a nitrogen molecule.

In this way, both nitrogen atoms attain inert gas (Ne) electronic configuration and complete their octet.

N + N 🡪 N₂

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How does Al form cation?

Answer:

Formation of Al⁺³ ion:

Aluminium belongs to group IIIA of the periodic table.

Aluminium has three outermost shell electrons.

To complete the octet, it loses its three outermost shell electrons.

Al 🡪 Al + 3e^–

(1s²,2s²,2p6,3s²,3p¹) (1s²,2s²,2p6)

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How does oxygen form an anion?

Formation of Anion from Oxygen:

Oxygen belongs to group VI A of the periodic table.

Oxygen has six valence shell electrons.

Oxygen needs two more electrons to complete its octet and achieve the noble gas configuration.

O + 2e^– 🡪 O ^-2

(1s²,2s²,2p⁴) (1s²,2s²,2p6)

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Write down the importance of noble gas electronic configuration.

Ans: Group VIIIA elements are named noble gases or inert gases.

These are helium(He),neon(Ne),argon(Ar),krypton(Kr),xenon(Xe) and radon(Rn).All the noble gases have eight valence shell electrons except helium(2 valence electrons).Due to complete octet,their ionization energy values are very high .That’s why noble gases are inert.

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What is the main distinction between ionic and covalent bonds? Explain your answer with suitable examples.

ANSWER :

Ionic Bond :

The bond which is formed by the complete transfer of electrons from one atom to another atom is called an ionic bond. Its also known as Electrovalent Bond.

CONDITION:

Following are the conditions that are necessary for the formation of ionic bonds.

• One element should be metal while the other elements should be non–metal.
• The electronegativity difference between two bonded atoms should be 1.7 or Above
• The ionization energy of metal should be low.
• The electronegativity of non-metal should be high

Bond Formation In NaCl :

1. Sodium belongs to group 1A
2. Chlorine belongs to group 7A
3. The electronegativity of sodium is 0.9.
4. While the electronegativity of chlorine is 3.0
5. The electronegative difference between Sodium (Na) and chlorine (Cl) is 2.1
6. Hence, an Ionic bond is formed
7. Sodium (Na) has one electron in its valance shell.
8. So, sodium (Na) loses one electron to form sodium ion (Na+) and attain the electronic configuration of (Neon)
9. Na 🡪 Na + 1e
10. Chlorine has seven electrons in its valance shell
11. So it gains one electron to form chloride ions (Cl^–) and attain the electronic configuration of argon
12. Cl + 1e 🡪 Cl
13. The electrostatic configuration of attraction is established on Na+ and Cl-
14. Therefore, crystal NaCl is formed

Na⁺ + Cl^– 🡪 Na – Cl

BOND FORMATION IN CALCIUM CHLORIDE

• Ca belongs to group 2A
• Cl belongs to group 7A
• The electronegativity of calcium (Ca) is 1.0 ,while Chloride (Cl) is 3.0
• The electronegativity difference between Ca and Cl is 2.0
• Thus, an Ionic bond is formed.
• Calcium has 2 electrons in its outermost shell
• It tends to lose the electrons and attain the inert gas electronic configuration of Neon (Ne) and form Ca+2

Ca 🡪 Ca+2 + 2e-

• Chlorine needs one electron to complete its outermost shell.
• It tends to gain one electron to attain the nearest noble gas electronic configuration.
• So it forms chloride ion (Cl-).
• 2Cl+ 2e- 🡪 2 Cl-
• Thus, the electrostatic force of attraction is established.
• Hence, white-coloured crystalline CaCl2 is formed.

Ca+2 + 2Cl 🡪 CaCl2

COVALENT BOND:

The chemical bond which is formed by the mutual sharing of electrons between atoms is called a covalent bond.

CONDITIONS FOR COVALENT BONDIG:

Both the elements should be non-metals.

The electronegativity difference between two bonded atoms should be less than 1.7.

Example:

O2

N2

CH4

CO2

FORMATION OF CL₂:

• A chlorine molecule is formed from two chlorine atoms.
• The chlorine atom electronic configuration is ( 2,8,7)
• A chlorine atom has seven electrons in its valence shell.
• The two chlorine atoms mutually share one electron to form a chlorine molecule (Cl2).
• Therefore, both chlorine atoms attain the electronic configuration of argon(Ar) and complete their octet.

Cl + Cl 🡪 Cl2

FORMATION OF CH₄:

Carbon has four electrons in its valence shell and needs more 4 electrons to attain the noble gas (Ne) configuration.

Therefore, 4 atoms of hydrogen mutually share one electron each with a carbon atom to form a molecule of methane (CH4).

C + 4H 🡪 CH₄

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Classify the following substances as covalent or Ionic.

1. NaCl
2. CO₂
3. MgCl₂
4. H₂O
5. C₆H₆
6. HCl

Answer:

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In what way do atoms form ions?

Answer:

Atom form ions due to loss or gain of electron/s.

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An ionic bond is formed between which two types of elements?

Answer:

An ionic bond is a chemical bond that is formed due to the complete transfer of electrons from one atom to another atom.

• One atom should be metal while the other is non-metal.
• The electronegativity difference should be 1.7 or above.

State the formula of ions formed by the following atoms?

Calcium 20:

• The atomic number of calcium is 20.
• It belongs to group IIA of the periodic table.
• Calcium is metal.
• The electronic configuration of Ca is

Based on the shell:

K=2

L=8

M=8

N=2

Based on sub-shell:

1s² , 2s² , 2p⁶ , 3s² , 3p⁶ , 4s²

According to the electronic configuration of ca, It losses two electrons to form calcium ion and attain the electronic configuration of noble gasses i.e Ar¹⁸

Ca _(2,8,8,2) 🡪 Ca⁺² _(2,8,8) + 2e

Florine F⁹ :

• The atomic number of F is 9
• It belongs to group VII A
• Hence, F is non-metal
• The electronic configuration of F is

Based on the shell:

K=2

L=7

Baesd on sub-shell:

1s² , 2s² , 2p⁵

According to the electronic configuration of F, it gains one electron to form fluorine ion and attain the electronic configuration of noble gases (Ne)

F + 1e^– 🡪 F^-1

(2,2,5) (2,2,6)

Lithium Li³:

• The atomic number of Lithium (Li) is 3.
• It belongs to group I A.
• Hence, Lithium is a metal.

The electronic configuration of Li is:

Based on Shell:

K= 2

L=1

Based on sub-shell:

1s²,2s¹

According to the electronic configuration of lithium, it loses one outermost shell electron to attain the electronic configuration of Helium (He).

Li 🡪 Li ⁺⁺ + 1e^–

^{(2,1) (2)}

Oxygen (O⁸):

The atomic number of oxygen is 8.

It belongs t group VI A.

The electronic configuration of oxygen is :

Based on Shell:

K=2

L=8

Based on Sub-shell:

1s2,2s2,2p4

According to the electronic configuration of oxygen,it gains 2 electrons to attain the electronic configuration of Neon(Ne).

O + 2e^– 🡪 O^-2

(2,6) (2,8)

Draw diagrams to show the ionic bond in MgO and CaCl2?

Diagram Representation of MgO :

Mg + O 🡪 MgO

Diagram Representation of MgCl2 :

Mg + 2Cl 🡪 CaCl2

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Define valence electrons?

Valence electrons:

The electrons in the outermost (valence) shell of an atom are called valence electrons.

The electrons which take part in bond formation are called bonding electrons and the electrons which do not take part in bond formation are called non-bonding electrons.

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Define valence shell?

Valence shell:

The outermost shell of an atom where the loss, gain, or sharing of electrons takes place during a chemical reaction.

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What is the importance of glues and adhesives in our society?

Ans: Glue is a substance that allows two surfaces to be bonded together.

The term is commonly used interchangeably with Adhesives.

An adhesive holds together the material.

There are different types of adhesives. e.g. cement. Glue etc.

There are many natural and synthetic adhesives in use.

• Adhesive can also be used as a coating on the surface that is subjected to corrosion and rusting.
• Adhesives are also used in many repair applications.
• These are commonly used to fix broken dishes and to make other repairs that would be impossible.
• This can also provide improved mechanical strength.
• A popular application of adhesive is to encapsulate electronic components, protecting that component against environmental and mechanical damage.

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