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Valence Bond Theory 🔴 for Class 11 in HINDI

In this Chemistry video in Hindi for class 11 we explained the valence bond theory. It was was introduced by Walter Heitler and Fritz London in 1927 and developed further by Linus Pauling and others. To understand this concept, let us consider the formation of hydrogen molecule which is the simplest of all molecules. Consider two hydrogen atoms A and B approaching each other having nuclei NA and NB and electrons present in them are represented by eA and eB. When the two atoms are at large distance from each other, there is no interaction between them. As these two atoms approach each other, new attractive and repulsive forces begin to operate. Attractive forces arise between: (i) nucleus of one atom and its own electron that is NA – eA and NB– eB. (ii) nucleus of one atom and electron of other atom i.e., NA– eB, NB– eA. Similarly repulsive forces arise between : (i) electrons of two atoms like eA – e B, (ii) nuclei of two atoms NA – NB. Attractive forces tend to bring the two atoms close to each other whereas repulsive forces tend to push them apart Experimentally it has been found that the magnitude of new attractive force is more than the new repulsive forces. As a result, two atoms approach each other and potential energy decreases. Ultimately a stage is reached where the net force of attraction balances the force of repulsion and system acquires minimum energy. At this stage two hydrogen atoms are said to be bonded together to form a stable molecule having the bond length of 74 pm. Since the energy gets released when the bond is formed between two hydrogen atoms, the hydrogen molecule is more stable than that of isolated hydrogen atoms. The energy so released is called as bond enthalpy, which is corresponding to minimum in the curve depicted in Fig. 4.8. Conversely, 435.8 kJ of energy is required to dissociate one mole of H2 molecule. Orbital Overlap Concept : In the formation of hydrogen molecule, there is a minimum energy state when two hydrogen atoms are so near that their atomic orbitals undergo partial interpenetration. This partial merging of atomic orbitals is called overlapping of atomic orbitals which results in the pairing of electrons. The extent of overlap decides the strength of a covalent bond. In general, greater the overlap the stronger is the bond formed between two atoms. Therefore, according to orbital overlap concept, the formation of a covalent bond between two atoms results by pairing of electrons present in the valence shell having opposite spins. Types of Overlapping and Nature of Covalent Bonds : The covalent bond may be classified into two types depending upon the types of overlapping: (i) Sigma bond, and (ii) pi bond Sigma-bond : This type of covalent bond is formed by the end to end (head-on) overlap of bonding orbitals along the internuclear axis. This is called as head on overlap or axial overlap. This can be formed by any one of the following types of combinations of atomic orbitals. • s-s overlapping : In this case, there is overlap of two half filled s-orbitals along the internuclear axis. • 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-bond : In the formation of bond the atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis. The orbitals formed due to sidewise overlapping consists of two saucer type charged clouds above and below the plane of the participating atoms. 👇👇👇👇👇👇 𝑷𝑳𝑨𝒀𝑳𝑰𝑺𝑻 👇👇👇👇👇👇 🔴 Full playlist on Chapter 4 of Chemistry : 'Chemical Bonding and Molecular Structure' for Class 11 :    • Chemistry 11th : Chemical Bonding and...   👇👇👇👇👇👇 Previous Video 👇👇👇👇👇👇 🔴 VSEPR Theory    • Valence Shell Electron Pair Repulsion...