MOT Flashcards
What is Valence Bond Theory?
Proposed by Heitler and London in 1927 and later developed by Pauling.
It is a theory which describes chemical bonding. VBT states that the overlap of incompletely filled atomic orbitals leads to the formation of a chemical bond between two atoms.
What are the salient features of VBT ?
SALIENT FEATURES ARE -
1) Covalent bond is formed due to the overlapping of half filled atomic orbitals of different atoms.
2) The overlapping atomic orbitals must have elections with opposite spin.
3) The inner orbitals of each atom remain undisturbed.
4) The strength of the covalent bond depends upon the extent of overlapping ; greater the overlapping , stronger will be the bond formed.
5) Depending upon the type of overlapping covalent bond may be sigma or pie .
What are the drawbacks of VBT?
1) Could not explain the geometry and shape of coordination compounds.
2) Could not explain the colour of coordination compounds.
3) Could not explain the paramagnetic behaviour of the oxygen atom.
4) Could not explain the magnetic moment.
5) Could not explain the odd e- formation.
What is Molecular Orbital Theory ?
Molecular Orbital Theory (MOT) explains how atoms combine to form molecules by using orbitals. In this theory, atomic orbitals (like those from individual atoms) mix together to form new orbitals called molecular orbitals that belong to the entire molecule.
Discuss the rules of MOT
1) MOT is based on the wave mechanics concept.
2) Atomic orbitals of the combining atom overlap and form molecular orbital therefore atomic orbitals lose their identity.
3) The molecular orbitals formed are equal to the number of atomic orbitals involved in their formation.
4) The filling of electrons in the molecular orbital is the same as that in atomic orbitals like Hund’s rule , Pauli’s rule and Aufbau rule.
5) Linear combination of atomic orbitals ( LCAD ) , according to this method the atomic orbitals of these atom ( eg - H ) are represented by wave function ( ¥ ) ¥a and ¥b for hydrogen atom A and B. When these atoms come closer and combine to form molecular orbitals.
H + H —-> H2 ¥a + ¥b —-> ¥ab ¥a ¥b
6) Wave function may be destructive or constructive and according to that the molecular orbital formed would be anti - bonding orbital and bonding molecular orbital respectively.
∅ + Ø ——> ∅∅ ( BMO )
∅ - Ø ——> ∅∅ ( ABMO )
Explain Graphite and its structure.
1) Graphite is an allotrope of carbon in which each carbon is sp² hybridised.
2) distance between layers = 3.41 Å and are held by weak Vander Val forces.
distance between C-C atoms = 1.42 Å
3) It consists of planer 2D dimensional sheets like structure made up of hexagonal networks
4) It is prepared by heating sand and coke in an electric furnace at 3300k temp.
5) The p orbital undergoes overlapping leading to the formation of π e- network.
6) In graphite each carbon uses only 3 valence e- , the 4th valence e- is free to move under the applied potential difference as a result graphic is a good conductor of electricity.
7) Density of graphite is 2.25 g/cc lesser than diamond (3.5 g/cc).
8) Coefficient of friction is low and small force is sufficient to slide the layer parallel to each other.
What are the applications of Graphite?
1) Pencils: Graphite is used as the main material in pencils, mixed with clay to create “lead.”
2) Lubricants: It works as a dry lubricant in machines because it reduces friction between moving parts.
3) Batteries: Graphite is used in the production of batteries, especially lithium-ion batteries.
4) Nuclear reactors: Graphite is used as a moderator to slow down neutrons in nuclear reactors.
What is fulleren ?
1) Fullerens are artificial allotropes of carbon. In fullerene compounds have a large number of carbon atoms in a molecule.
2) Fullerene was discovered by Krotto & Smalley in 1985 at Rice University USA.
3) It is also called as bick minister fullerens in honour of American architecture Buck Minister fuller who designed a structure i.e Geodeic dome structure.
Write down the preparation of fullerens
It is prepared by the vaporization of graphite electrode in an inert Atmosphere to prevent oxidisation.
Mixture of fullerens is obtained, it is separated by column chromatography and solvent extraction.
Explain the Structure & properties of Fullerens.
1) Truncated Icosahedron
2) Icosahedron is a polygon with 60 vertices and 32 faces out of which 12 are pentagons and 20 are hexagons also called soccer ball ( football )
( STRUCTURE )
3) Each carbon is sp² hybridised
4) Fullerene is black powdery material
5) It forms deep magenta colour when dissolved in benzene ( organic solvents )
6) It is tough and thermally stable material.
Applications of Fullerens
1) It is used as a lubricant due to its spherical structure.
2) It is used in electronics & micro-electronic devices.
3) It is used as a superconductor, when mixed with alkali.
4) It is used in solar cells . They can minimise the photosynthetic process of plants by accepting electrons.
5) They can also be used as polymer additives.
