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Limitations Of Valence Bond Theory Pdf __EXCLUSIVE__ Download


Valence bond theory provides an explanation for the structure and magnetic characteristics of a large number of coordination compounds. The valence bond theory was used to explain the structure of coordination molecules and their bond connections.




limitations of valence bond theory pdf download



A covalent link between two atoms is produced by the overlap of their half-filled valence atomic orbitals, each of which contains one unpaired electron. While a valence bond structure is similar to a Lewis structure, it is employed in situations where a single Lewis structure cannot be written. Each of these VB structures is a Lewis structure in its own right. The primary focus of resonance theory is on this combination of valence bond configurations. According to the valence bond theory, a chemical bond is formed when the overlapping atomic orbitals of the involved atoms. Due to the overlapping, electrons are most likely to be in the bond region. Bonds are viewed as weakly connected orbitals in the Valence bond theory (small overlap). In general, valence bond theory is easier to apply to ground-state molecules. During bond formation, the core orbitals and electrons stay basically unaltered.


The requirement of maximum overlap is a critical feature of the valence bond theory, as it results in the production of the strongest possible bonds. This theory is used to explain the development of covalent bonds in a wide variety of compounds.


The Valence Bond Theory (VBT) examines the interaction of atoms in order to explain chemical bonding. It is one of two important ideas that contribute to our understanding of how atoms combine. The valence bond hypothesis explains how covalent bonds are formed. Additionally, it assists in determining the electrical structure of molecules. Additionally, one can explain the shape of an atom in a molecule using VBT and hybridization. VBT, on the other hand, is unable to account for the existence of inner and outer orbital complexes.


Ans. The valence bond theory explains how covalent bonds are formed and the electronic structure of molecules. The hypothesis postulates that electrons inhabit the atomic orbitals of individual atoms inside a molecule and that electrons from one atom are attracted to the nucleus of another.


In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds when a molecule is formed. In contrast, molecular orbital theory has orbitals that cover the whole molecule.[1]


In 1916, G. N. Lewis proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as Lewis structures. The chemist Charles Rugeley Bury suggested in 1921 that eight and eighteen electrons in a shell form stable configurations. Bury proposed that the electron configurations in transitional elements depended upon the valence electrons in their outer shell.[3] In 1916, Kossel put forth his theory of the ionic chemical bond (octet rule), also independently advanced in the same year by Gilbert N. Lewis.[4][5] Walther Kossel put forward a theory similar to Lewis' only his model assumed complete transfers of electrons between atoms, and was thus a model of ionic bonding. Both Lewis and Kossel structured their bonding models on that of Abegg's rule (1904).


Linus Pauling published in 1931 his landmark paper on valence bond theory: "On the Nature of the Chemical Bond". Building on this article, Pauling's 1939 textbook: On the Nature of the Chemical Bond would become what some have called the bible of modern chemistry. This book helped experimental chemists to understand the impact of quantum theory on chemistry. However, the later edition in 1959 failed to adequately address the problems that appeared to be better understood by molecular orbital theory. The impact of valence theory declined during the 1960s and 1970s as molecular orbital theory grew in usefulness as it was implemented in large digital computer programs. Since the 1980s, the more difficult problems, of implementing valence bond theory into computer programs, have been solved largely, and valence bond theory has seen a resurgence.


According to this theory a covalent bond is formed between two atoms by the overlap of half filled valence atomic orbitals of each atom containing one unpaired electron. A valence bond structure is similar to a Lewis structure, but where a single Lewis structure cannot be written, several valence bond structures are used. Each of these VB structures represents a specific Lewis structure. This combination of valence bond structures is the main point of resonance theory. Valence bond theory considers that the overlapping atomic orbitals of the participating atoms form a chemical bond. Because of the overlapping, it is most probable that electrons should be in the bond region. Valence bond theory views bonds as weakly coupled orbitals (small overlap). Valence bond theory is typically easier to employ in ground state molecules. The core orbitals and electrons remain essentially unchanged during the formation of bonds.


An important aspect of the valence bond theory is the condition of maximum overlap, which leads to the formation of the strongest possible bonds. This theory is used to explain the covalent bond formation in many molecules.


Valence Bond Theory accounts for the directional nature of the covalent bonds. In this article, we are going to learn all about Valence Bond Theory, Postulates of Valence Bond Theory, Interacting Forces in Covalent Bond Formation, Potential Energy Diagram, Formation of \(H_2\) molecule, Importance of Valence Bond Theory, and Limitations of Valence bond theory


This theory was proposed by Heitler and London in the year 1927 on the basis of a wave mechanics of electrons and it was further extended by Pauling and Slater in the year 1931. According to this theory, a covalent bond is formed between two atoms by the overlap of half-filled valence atomic orbitals of each atom containing one unpaired electron. This theory accounts for the directional nature of the covalent bonds.


Valence bond theory only explains the formation of a covalent bond in which a shared pair of electrons comes from two bonding atoms. However, it offers no explanation for the formation of a coordinate covalent bond in which both the electrons are contributed by one of the bonded atoms.


Valence bond theory does not explain the bonding in electron-deficient molecules like \(B_2H_6\), in which the central atom possesses a fewer number of electrons paramagnetism of oxygen molecules than required for an octet of electrons.


Understanding the nature of bonding is vital for studying coordination compounds. The Valence Bond theory explains chemical bonding using quantum mechanics. What is valence bond theory? A description of individual bond formation from the atomic orbitals of the participating atoms during a molecule formation is referred to as VB theory.


The VB theory predicts that there are no unpaired electrons in molecular oxygen. The forms of covalent compounds are well qualitatively described by VB theory. In contrast, the Molecular Orbital (MO) hypothesis is useful for comprehending bonds more generally.


The fusion of atomic orbitals to form new hybridized orbitals is called hybridisation. It influences bonding properties and molecular geometry. The concept of hybridisation originates from the valence bond theory. As per the valence bond theory, a metal atom/ion influenced by a ligand uses its (n-1)d, ns, np, or ns, np, nd orbitals to give rise to certain equivalent orbitals of definite geometry. These hybrid orbitals overlap with ligand orbitals that donate electron pairs for bonding.


The valence-shell orbitals 4s and 4p are empty. Thus, the 3d electrons are in the dxy, dxz, and dyz orbitals. The mixing of 4s, 4px, 4py, 3dx2-y2, and 3dz2 orbitals form empty d2sp3 orbitals that point towards the octahedron corners. Each orbital can accept a non-bonding pair of electrons from an NH3 molecule for a complex formation.


Five years after the VB theory, the molecular orbital theory came forward to explain the bonding in molecules that the VBT could not. The following table states the main differences between VBT and MOT:


Both VSEPR and VBT are applied to compounds with covalent bonds. VBT is considered superior to VSEPR because VSEPR theory only explains the shape of a molecule, while VBT tells about the creation of covalent bonds between atoms of a molecule.


Valence Bond Theory was for the first time proposed by Hietler and London and then modified by Pauling and Slater. It is based upon the pairing of electrons and the neutralization of their spins. The main postulates of this theory are as under-(1) The covalent bonds are formed by overlapping of atomic orbitals, but in molecule atomic orbitals do not lose their identity. Thus, shape, size, and other parameters of atomic orbitals remain unchanged in the molecule. Thus, in molecules, the electrons are found in atomic orbitals. Hence, this theory is also called Atomic Orbital Theory (AOT). (2) In overlapping, there occurs pairing of electrons and neutralization of their spins.(3) The strength of a covalent bond depends upon the extent of overlapping between the atomic orbitals. The larger the extent of Overlapping, the stronger is the bond formed.


This theory successfully explains the formation of molecules like H2, Cl2, NH3, CH4, etc. However, it has the following limitations-(1) It could not explain the formation of odd election molecule ions like H2+ molecule ion.(2) It could not explain the magnetic behavior of molecules. For example, according to this theory, the O2 molecule should not contain any unpaired election and therefore, it should be diamagnetic in nature. However, in practice, it is found to be paramagnetic.(3) It does not give any idea about resonance.


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