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Briefly describe the valence bond theory of covalent bond formation by taking an example of hydrogen. How can you interpret chergy changes taking place in the formation of dihydrogen ? |
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Answer» Solution :VALENCE bond theory (VBT) was introduced by Heitler and London (1927) and developed further by Pauling and otheL VBT is based on the knowledge of atomic orbitals, electronic configurations of elements, the overlap criteri~ of atomic orbitals, the hybridization of atomic orbitals and the principles of variation and superpositfon. Consider two hydrogen atoms A and B approaching each other having nuclei `N_(A) and N_(B)` and electrons present in them are represented by `e_(A) and e_(B)`. 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 : i.e., `N_(A) - e_(A) and N_(B) - e_(B)` (ii) nucleus of one atom and electron of other atom : i.e.,`N_(A) - e_(B) and N_(B) - e_(A)` Similarly, repulsive forces arise between (i) electrons of two atoms like `e_(A) - e_(B)`(ii) nuclei of two atoms like `N_(A) - N_(B)` Attractive forces keep the two atoms close to each other and repulsive forces TRY to push them away. Experimentally, It is 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. So, a stage is reached where the NET force of attraction balances the force of repulsion and system attain minimum energy. At this stage, two H-atoms are said to be bonded together to form a stable molecule having the bond length of 74 pm.  The energy gets released when the bond is formed between two hydrogen atoms, the hydrogen molecule is more stable than that ofisolated hydrogen atoms. The energy so released is called as bond enthalpy, which is corresponding to minimum in the curve depicted in the given figure. CONVERSELY 435.8 kJ of energy is required to dissociate one mole of `H_(2)` molecule. `H_(2(g)) = 435.8 " kJ mol"^(-1) rarr H_((g)) + H_((g))`  The potential energy curve for the formation of `H_(2)` molecule as a function of internuclear distance of the H-atoms. THe minimum in the corve corresponds to the most stable state or `H_(2)`. |
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