COVALENT BONDING
Covalent Bonding:
“The Bonding in which sharing of valance electrons takes place by atoms in order to stabilize their outer shell Electronic configuration.”
Sharing of electrons results in lowering of potential energy of system.
Explanation:
Ø Two hydrogen atoms contribute their 1s1 electron to form an electron pair covalent bond.
Single Covalent Bond:
A covalent bond that is formed due to sharing of a single pair of electrons.
Ex: Hydrogen molecule (H2)
Double Covalent Bond:
A covalent bond that is formed due to sharing of a two pair of electrons.
Ex: Oxygen molecule (O2)
Triple Covalent Bond:
A covalent bond that is formed due to sharing of a three pair of electrons.
Ex: Nitrogen molecule (N2)
Ø Covalent bond may be formed when two or more atoms of same electrons share one or more electron pairs of valance electrons.
Ex: H2, O2, N2
Ø Covalent bond may also be formed when two or more atoms of different non-metals share one or more pairs of valance electrons.
Ex: H2O, C2O, HCl and NH3
Formation of Covalent Bond in H2:
Ø When two atoms come together their electron cloud interacts and overlaps.
Ø The probability of finding the 1s1 electron in atoms is maximum between the two nuclei of atoms.
Ø In order to comply with Pauli’s exclusion principle shared two electrons should be of opposite spins.
Ø The interaction between these two anti spin electrons gives rise to force of attraction between the two atoms.
Ø When the electrostatic repulsion between the atoms is balanced by this force of attraction at equilibrium separation the molecule becomes stable.
Ø The potential energy under this condition is minimum and the bond between the Hydrogen atoms is saturated.
Ø The directional nature of the covalent bond results from restricted orbital motion of electrons.
Hybrid Bonding:
“Covalent bonds are formed not only due to overlap of pure s-orbitals or p-orbitals but also due to overlap of s-p orbitals, such bond is called hybrid bonding.
Ex: Normal carbon exhibits such bonding.
Ø Electronic configuration of normal carbon atom is 1s2 2s2 2p2
Ø Electron spin distribution of normal carbon atom is
↑↓ ↑↓ ↑ ↑
1s2 2s2 2px1 2py1 2pz
Ø The unpaired electrons are responsible for bond formation. Two bonds are expected to form.
Ø When two carbon atoms approach each other one of the electron in 2s orbital gets excited to 2p orbital.
Ø Now spin distribution in each carbon atoms is
↑↓ ↑↓ ↑ ↑ ↑
1s2 2s2 2px1 2py1 2pz1
Ø This gives rise to four unpaired electrons.
Ø 4 bonds are expected to form
SP3 Hybridization:
Ø The bonding direction of 4 orbitals are directed towards the four corners of regular tetrahydron bond angle is 109.5o
Ø The four orbitals are called SP3 hybrids.
Ø Arrangement of these orbitals is hybridization.
Ex: Diamond
ü Because of strong directional primary valance forces diamond becomes strong with high melting point and low thermal expansion co-efficient
ü Since it is extremely hard, it is used as an abrasive.
ü Since valance electrons are strongly inter locked in covalent bond it is an insulator
ü Silicon and Germanium are electrical insulators at 0ok and they are electrical conductors when temperature is raised. So, these are called semi-conductors.
SP2 Hybridization:
Ø Graphite results from SP2 hybridization.
Ø Three SP2 hybrid covalent bonds are formed in a plane with bond angle 120o
Ø The fourth p-orbital forms a perpendicular bond (Vander wall bond) in a direction perpendicular to SP2 bonds.
Ø The Vander wall bond is weak. So structure of graphite is layered.
Ø Since carbon atoms with in a plane (layer) are held together by strong covalent bonds. The conductivity with in a layer is low.
Ø Due to weak nature of Vander Wall bonds. There is large conductivity between layers in perpendicular direction.
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