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Chemical Bonding

Chapter 4: Molecular Structure & Valency

Chemistry ki duniya mein atoms akele rehne ke bajaye aksar judkar molecules banana pasand karte hain. Is chapter "Chemical Bonding and Molecular Structure" mein hum unhi taakaton (forces) ke baare mein padhenge jo atoms ko ek saath baandh kar rakhti hain. Aaj hum shuruat karenge basic concepts jaise Valency aur Kossel-Lewis Theory se, jo chemistry ko samajhne ke liye bahut zaroori hain.

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Chapter – 4
Chemical Bonding and Molecular Structure

Valency:

Greek word Valentia that means power or capacity, so “combining power of one element with other is known as Valency”.

Electronic Theory of Valency:

This theory was proposed by Kossel and Lewis. According to this theory —

• (i) The outermost shell of element is known as valence shell.
• (ii) The occurring of 8 electron in outermost shell is stable state that is known as octet.
  
  For eg. Group 18 element

  • He = 2
  • Ne = 2, 8
  • Ar = 2, 8, 8
  • Kr = 2, 8, 18, 8
• (iii) To complete octet remains element combine to another.
• (iv) The element that have 1, 2, 3 electron in their outermost shell they have loosing tendency of e⁻.
• (v) The element that have 5, 6, 7 electron in their outermost shell they have gaining tendency of electron.

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(vi) On the basis of valency there are 3 types of chemical bond —

1. Electrovalent Bond / Ionic Bond
2. Covalent Bond
3. Coordinate Bond

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1. Electrovalent Bond:

The bond that are formed by complete transfer of electron is known as electrovalent bond. These bond are form by in between of ion so, it is also called ionic bond.

For eg.:

(i) Formation of NaCl

₁₁Na = 2, 8, 1
₁₇Cl = 2, 8, 7

Na + Cl → Na⁺ + Cl^-   or   Na⁺Cl^-

(ii) Formation of MgO

₁₂Mg = 2, 8, 2
₈O = 2, 6

Mg + O → Mg²⁺ + O^2-   or   MgO

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(iii) formation of CaCl₂

₂₀Ca = 2, 8, 8, 2          ₁₇Cl = 2, 8, 7

               :Cl:
Ca** +   ↗
              ↘
               :Cl:

→

Ca⁺⁺ + 2[:Cl:]^- or Ca⁺⁺2Cl^-

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Conditions for formation of electrovalent bond

• * one atom should be greater size and another should be smaller size.
• * one atom can easily loose electron and another should be can easily gain electron.
• * one atom should be metal and another should be non-metal.
• * one atom should be highly electropositive and another should be highly electronegative.
• * They have the difference of electron 1, 2, 3 unit to complete their octet.

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Characteristics of electrovalent compound

(HOH water)

(i) They are soluble in polar solvent and insoluble in non-polar solvent.
(alcohol)

(ii) Melting and boiling point of electrovalent compound is very high.

(iii) They can conduct electricity in aqueous solution or molten state.

(iv) Electrovalent Bond are non-directional.

(V) These compound are generally solid in physical state.

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Co-valent Bond:-

The bond that are formed by equal no. of sharing of electron between two atom are called co-valent bond.

The compound that are formed by co-valent bond are called co-valent compound.

for example—

1 formation of H₂

₁H = 1
(H•   •H) → H — H   or   H₂

2 formation of Cl₂

₁₇Cl = 2, 8, 7
(:Cl: :Cl:) → :Cl• — •Cl:   or   Cl₂

3 formation of O₂

₈O = 2, 6
(:O:: :O:) → :O = O:   or   O₂

4 formation of N₂:-

₇N = 2, 5
(:N::: :N:) → :N ≡ N:   or   N₂

         

5 Formation of CH₄

₆C = 2, 4
₁H = 1

  H•
  ↓
H• → C ← •H
  ↑
  H•

→

  H
  |
H — C — H
  |
  H

In the formation of co-ordinate bond one atom or group already completed their octet and donate the loan pair of electron to another atom or group.

co-ordinate bond is represented by arrow sign the direction of arrow will be dooner to acceptor.

for. e.g.      formation of O

₈O = 2, 6

:O = O → :O:

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formation of SO₂

₁₆S = 2, 8, 6

₈O = 2, 6

:O = S → :O:

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Conditions for formation of Co-ordinate bond:-

* Both e^- donate by single atom or group

* one atom or group already complete their octet.

* one atom or group can donate the loan pair electron and and another atom or group can accept the loan pair electronc haracteristics of co-ordinate bond.

⑤ Formation of CH₄ (Methane)

₆C = 2, 4
₁H = 1

  H•
  ↓
H• → C ← •H
  ↑
  H•

→

  H
  |
H — C — H
  |
  H

Co-ordinate Bond

In the formation of co-ordinate bond one atom or group already completed their octet and donate the loan pair of electron to another atom or group.

co-ordinate bond is represented by arrow sign the direction of arrow will be dooner to acceptor.

for. e.g.      formation of O₃

₈O = 2, 6

:O = O → :O:

formation of SO₂

₁₆S = 2, 8, 6

₈O = 2, 6

:O = S → :O:

Conditions for formation of Co-ordinate bond:-

* Both e^- donate by single atom or group

* one atom or group already complete their octet.

* one atom or group can donate the loan pair electron and and another atom or group can accept the loan pair electron

characteristics of co-ordinate bond.

Molecule	Electron dot structure	Bonding structure
H2O	.. H :O: H ..	H — O — H
NH4Cl	H .. H :N: H [:Cl:]- .. H	⎡ H ⎤ ⎢ | ⎥ ⎢ H—N—H ⎥ Cl- ⎢ | ⎥ ⎣ H ⎦
C2H5OH	H H .. .. .. H:C : C : O:H .. .. .. H H	H H | | H—C — C — O—H | | H H
KCN	K+ [:C:::N:]-	K+ — C ≡ N
H2SO4	:O: .. : .. H:O: S :O:H .. : .. :O:	O ↑ H — O — S — O — H ↓ O

(1)

Polar Covalent Bond:-

formed by which have electronegativity difference more than 1.7 , The more electronegativity atom attract more bond become to the shared pair electron , so co-valant bond become polar that is known as polar covalant bond

for. e.g. - formation of HCl

H * :Cl:    —    H^+δ — Cl^-δ

Due to polarity of Hydrogen & Chlorine Bond they can easily ionise in Aqeous solyn and can conduct electricity.

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Sigma (σ) Bond And Pie (π) Bond

The incomplete orbital of atom have greater tendancy to complete their halffill atomic orbital , in this way they come close contact to each other and pair their halffilled orbitals and shows following type of bond.

Sigma Bond:-

The bond that are formed by Head or Axial overlapping of two atomic orbital are known as sigma bond.

These bond are formed by three ways:-

* By s-s overlapping of two atomic orbital

* By s-p overlapping of two atomic orbital.