Lewis structure

INTRODUCTION

The attractive force which holds together the constituent particles (atoms, ions or molecules) in chemical species is known as chemical bond.

1. Kössel-Lewis Approach to Chemical Bonding

They assumed that atom have positive kernel surrounded by electrons occupying the corners of a cube. If they have all the eight electrons in their outer shell they will be stable (octet rule). Otherwise they achieve stability (octet) through chemical bonding.

     LEWIS STRUCTURE

Why chemical bonds are formed.

If the resultant molecule has lower Gibb's energy then the reacting species, then chemical bonds are formed.

IONIC BOND

Ionic bonds are strong electrostatic forces between cation & anion which are formed when an atom looses an electron or gains an electron.

 

BORN HABER CYCLE FOR SODIUM CHLORIDE

    

Ex.1:  Draw Born Haber cycle for formation of Magnesium chloride

 

PROPERTIES OF IONIC COMPOUNDS

1. They are crystalline in nature
2. They have high Melting and Boiling point
3. Hard and Brittle
4. Soluble in polar solvents
5. Conduct electricity in molten state and aqueous state but not in solid state.
6. Do not show Isomerism.

VARIABLE ELECTROVALENCY

Fe (26) → 3s² 3p⁶ 3d⁶ 4s²

Fe⁺² (24) → 3s² 3p⁶ 3d⁶ (less stable)

Fe⁺³ (23) → 3s² 3p⁶ 3d⁵ (more stable)

Solubility of ionic compounds in water. There are two things happen on dissolving ionic compounds in water.

1. Breaking of ionic lattice

2. Mixing of ions in water

For the first process, lattice energy to be provided to the solution and for second process hydration energy will be released by the system

If ΔH_hyd >ΔH_L.E. Compound is soluble in water

(a) Lattice Energy

1. If depends on size of cation and anion.

Smaller the size greater is the Lattice Energy.

Ex. → LiCl > NaCl > KCl

      → NaCl > NaBr > NaI

2. If depends on Charge of cation and anion.

Bigger the charge, lesser is it Lattice energy.

Ex. MgCl₂ > NaCl

3. In case anions are extremely large, then the rule one charges smaller the cation lesser is Lattice Energy.

Ex. MgSO₄ < CaSO₄ < SrSO₃ < BaSO₄

(b) Hydrogen energy

Smaller the cation, greater is the hydration enthalpy.

Ex. Compare Solubility BaSO₄ and CaSO₄

L.E. Þ BaSO₄ > CaSO₄

Hyd. Energy Þ CaSO₄ > BaSO₄

So CaSO₄ is more soluble.

Ex. Compare solubility of NaCl and BaCl₂

L.E. Þ BaCl₂ > NaCl

Hyd. Energy Þ NaCl > BaCl₂

So NaCl is more soluble.

 

COVALENT BOND

Combining of unpaired electrons of atoms to achieve a stable configuration and formation of molecules is called covalent bond.

SIGMA (s) BOND: THIS TYPE OF COVALENT BOND is formed by the end to end (head-on) overlap of bonding orbitals along the inter-nuclear axis. This is called as head on overlap or axial overlap

s - s overlapping: In this case, there is overlap of two half killed s-orbitals along the inter-nuclear axis as shown below:

 

s-p overlapping: This type of overlap occurs between half filled s-orbitals of one atom and half filled p-orbitals of another atom:

 

p-p overlapping: This type of overlap takes place between half filled p-orbitals of the two approaching atoms:

 

Pi (π) BOND: IN THE FORMATION OF π BOND

The atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the inter-nuclear axis.

 

Characteristic of Convent compounds

1. Physical state ® gases, liquids of law b.p. & soft solids
2. Melting Point/Boiling point ®  with exception of network solids, they have low M.P. and B.P.
3. Electrical conductance ®  generally bad conductors
4. Solubility  ®  soluble in non-polar solvents
5. Isomerism  ®  yes Physical state ® gases, liquids of law b.p. & soft solids
6. Melting Point/Boiling point ®  with exception of network solids, they have low MP/B.
7. Electrical conduce ®  generally bed conductors
8. Solubility  ®  soluble in non-polar solvents
9. Isomerises  ®  yes

COMPARISON BETWEEN IONIC AND COVALENT BONDS

COMPARISON BETWEEN IONIC AND COVALENT COMPOUNDS

Some representative Bond Energies and Bond Lengths