UNIT – I
CHAPTER – I
BONDING IN SOLIDS
Introduction:
Performance of the material depends on properties of solid.
Properties of a material depend on internal structure of a material.
Solid:
Close the collection of atoms results in bulk materials called solids,
Solid is classified in to two types Crystalline and Non- Crystalline.
Ø An atom consists of positively charged nucleus surrounded by negatively charged electron cloud
Ø When two atoms are brought closer both attractive and repulsive forces acted upon.
Ø When attractive force balance with repulsive force Fatt = Frep. They take relative positions.
Ø The attractive and repulsive forces are responsible for bond formation
Ø The attractive force are directly associated with valance electron
Ø Outer shell contains valance electrons
Ø Outer shell can acquire more electron (or) less electron to another atom to become more stable
Ø An atom can share electrons with two (or) More atoms
Ø In this way atomic bonds are formed.
Equilibrium Distance:
Ø The distance between two atoms where attractive and repulsive force must balance with each other to form a bond.
Ø At Equilibrium distance attractive force is equal to repulsive force
Ø At equilibrium distance total energy of the system is less
Ø At equilibrium distance bond is formed
Cohesive Energy:
Ø The energy released when two atoms brought from infinite to closer proximity to form a bond.
Energy of Dissociation:
Ø The energy required to separate the two atoms from equilibrium Distance to infinite.
CLASSIFICATION OF BOND:
Ø Based on different charge distribution in atom ‘s they are classified in to five Types
1. Ionic Bond
2. Covalent Bond
3. Metallic Bond
4. Vander wall bond
5. Hydrogen Bond
Ø Based on bond strength and directionality they are classified in to two types.
1. Primary Bond
a. Ionic Bond
b. Covalent Bond
c. Metallic Bond
ü Primary bonds are formed by virtue of their nature and inter atomic bonds
ü They are Strong bonds
ü Bond energy of primary bond is 0.1eV/bond to 10eV/bond
2. Secondary Bond
a. Vander wall Bond
b. Hydrogen Bond
ü Secondary Bonds are weak bonds
ü Bond energy of secondary bonds is 0.01eV/bond to 0.5eV/bond
Ionic Bonding:
Ø An attractive force between positively charged ion and negatively charged ion when they are brought to close proximity. These ions are formed when the atoms of different element involved in lose or gain of electron in order to stabilize the outer shell electronic configuration.
Ø Electro positive elements readily give up electrons
Ex: I and II Group elements.
Na, K, Ba
Ø Electro Negative elements readily take up electrons.
Ex: V and VI group elements
Cl, Br, O
Explanation:
Ø Na and Cl atoms are brought together.
Ø One valance electron gets transform from Na atom to Cl atom acquires stable electronic configuration.
Ø Now cation Na+ and anion Cl- are formed
Ø There Exists a electrostatic attraction between positively charged Na+ cation and Negatively charged Cl- anion
Ø Now ions are brought to Equilibrium distance,
Ø Ionic Bond is Formed
Properties of Ionic Bonding:
Ø Ionic solids are crystal in nature.
Ø Melting point varies with charge an ion because strong binding force and ionic radius.
Ø Stable in polar solvents due to large electrostatic interaction between polar solvents and ions. However insoluble in non-polar solvents.
Ø Ionic solids are hard and brittle.
Ø Ionic solids are non-directional because charge distribution is spherical in nature.
Ø Ionic solids are good insulators. Because of high band gap i.e.
o ρ of Alkali halides is 1012 Ω -cm.
o ρ of Al2 O3 is 1016 Ω at room temperature.
o ρ of Ionic solids decreases at room temperature.
o TCR (Temperature co-efficient of receptivity) of ionic solids is negative.
Ø In molten states (High temp.) Ionic solids are conductors of electricity.
Ø At High Temperature electrical conductivity (σ) is due to mobility (μ) of ions.
Ø Addition of impurities to ionic crystal strongly effect conductivity. This Property is useful in solid state of batteries.
Ø Ionic bond is strong bond
Ø Ionic Solids have strong binding energy
Ø Transparent to visible light
Ø Ionic solids have closely packed structure
Ø Ionic crystal are rigid
Ø Few ionic solids
o Alkali halides are used in infrared filters because they have absorption bands in IR radiation.
Ø Few ionic solids
o Alkali halides when subjected to x-rays and γ-rays they get colored because they absorb visible radiation this absorption is due to color centers.
Ø For MF, MCl, MBr , MI
Factors influencing the formation of Ionic bond.
Ø Low ionization energy and
Ø High electron affinity
Ø Large lattice energy
Bond energy of NaCl Molecule:
Let us consider Na and Cl atoms at large separation.
Ionization Energy:
Ø The energy required to remove an electron from the outer most shell of an isolated gaseous atom
ü I.E. of Na atom is 5.1eV
Na + 5.1eV à Na+ + e-
ü Here Na atom becomes Na+ Cation.
Electron affinity:
Ø The energy released when an electron is added to isolated gaseous atom is called electron affinity.
Cl + e- à Cl- + 3.6eV
ü Here Cl atoms becomes Cl- anion
Ø An electrostatic attraction between Na+ and Cl- ions bring closer to the equilibrium position. Potential energy is minimum. The energy released due to the formation of NaCl molecule is
Ø The Net energy is released when Nacl Molecule is formed from neutral Na and Cl atoms is bond Energy,
§ B.E. = E.A. + I.P. + V
§ B.E. = – 3.6 + 5.1 – 6
§ B.E. = – 4.5 eV
Ø So, to dissociate NaCl Molecule in to Na and Cl ions it requires 4.5eV (energy of Dissociation)
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