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Molecular Orbital Theory To describe the covalent bond formation and nature of electron sharing, two theories have been proposed: Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT). In Valence Bond Theory , only the half filled orbitals of valence shell take part in bond formation and the remaining orbitals retain their identity. But Molecular Orbital Theory (MOT) suggests the combination of all atomic orbitals having comparable energy and proper symmetry. Molecular Orbital Theory (MOT) was developed by F. Hund and R.S Mulliken in 1932. Main postulates of this theory are : 1. Atomic orbitals of comparable energy and proper symmetry combine together to form molecular orbitals. 2. The movement of electrons in a molecular orbital is influenced by all the nuclei of combining atoms. (Molecular orbital is poly centric in nature) 3. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals (AO's) combine t...

Definition of Heisenberg's Uncertainty Principle According to Heisenberg's uncertainty principle, it is not possible to determine precisely both the position and momentum (or velocity) of a moving microscopic particle, simultaneously with accuracy. Mathematical Expression of Heisenberg's Uncertainty Principle ▵x .▵p => h / 4π Where  ▵x is uncertainty with regard to the position and ▵p is uncertainty with regard to the momentum of the particle. If ▵x is very small ▵p would be large , that is , uncertainty with regard to momentum will be large. On the other side if we attempt to find out the momentum exactly the uncertainty with regard to position will be large. Explanation of   Heisenberg's Uncertainty Principle To determine the position of a small body like electron, it has to be illuminated with electromagnetic radiation. Low energy radiations like ordinary light waves cannot be used to illuminate a small body like electron, since the size of the ...

De-Broglie Hypothesis In 1924, de-Broglie proposed that matter has a dual character , as wave and as particle . In Bohr theory, electron is treated as particle. But according to De-Broglie, electron has a dual dual character; both as a material particle and as a wave. He derived an expression for calculating the wave length 'λ' of a particle of mass 'm'  moving with velocity 'v'. According to this, wave length = λ =  h / mv  , where 'h' is Planck's constant This is equation is known as De-Broglie's equation and it is an expression for wave - matter dualism . The waves associated with particles in motion are called matter waves or De-Broglie waves . They differ from electromagnetic radiations. They have lower velocities, and no electrical and magnetical fields associated with them. Derivation of  De-Broglie's equation The de-Broglie's equation can be derived by using the mass energy relationship suggest...

Quantum numbers are the address of electrons. These are numbers used to specify the position and energy of electrons in an atom. Four quantum numbers are usually used to designate the electrons present in an orbital. 1. Principal quantum number(n) 2. Azimuthal quantum number(l) 3. Magnetic quantum number(m) 4. Spin quantum number(s) 1. Principal quantum number(n) This represents the main shells in an atom. It determines the size and energy of orbitals. It can have any positive value from 1 to infinity. Shells are designated as K, L, M, N etc., When n= 1, 2, 3, 4....etc respectively. 2. Azimuthal quantum number(l) This represents the sub shell. It determines the shape of orbitals It also gives the orbital angular momentum quantum number. 'l' may have values from 0 to (n-1). If the principal quantum number n=1, 'l' has a value of '0', there will be only 's' subshell. 3. Magnetic quantum number(m) This represents the orbitals. ie, the orientation of orbital...