Valence Bond Theory
The electrostatic attraction between the positive and the negative ion that holds an ionic bond together makes it very simple to understand the concept of ionic bonding easier.
This is not the case with the covalent bonding. We know that a covalent bond is formed by mutual sharing of electrons. But, how and why the sharing happens, what does it do to the geometry of the molecules bonded covalently, is a mystery.
It was only after the development of wave mechanics that it was to a greater extent solved by theories and concepts like valence bond theory, VSEPR theory and molecular orbital theory.
Valence Bond Theory
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This theory is due to Heitler London, Pauling and Slater and was first applied in 1927 to hydrogen molecule. In case of hydrogen molecule, we consider the possibility of interchange of electrons between the two H atoms.
Although the two H- atoms in a Hydrogen molecule are identical, for convenience of treatment, let us label them as HA and HB. Let these two atoms, HA and HB is at an infinite distance from each other so that no appreciable interaction may occur. Thus, there are two possible equivalent structures of Hydrogen molecule, (I and II as shown below) which are indistinguishable once the bond has been formed. 1HA 2HB 2HA 1HB
In the structure (I) we have the nucleus HA with electron 1 and nucleus HB with electron 2. In structure (II), the electrons have been exchanged.
If ψ(I) and ψ(II) are the wave functions for the structure (I) and (II) respectively, then according to Valence bond theory. ψI = ψA (1) ψB (2) and
ψII = ψA (2) ψB (1)
ψA (1), ψB (2), ψA (2) and ψB (1) are wave functions for the independent H-atoms. Postulates of Valence bond theory
The overlap of atomic orbitals is given by the following important postulates of Valence bond theory. 1.
The atoms which unite to form a molecule completely retain their identities in the resulting molecule. 2.
The formation of a covalent bond is due to overlap of atomic orbitals (abbreviated to AO’s ). If the two atoms, each having on unpaired electron, come together, the AO’s accommodating these unpaired electrons overlap (i.e, electron waves interact) and the spins of the two electrons get mutually neutralized, resulting in the formation of covalent bond which is localized between the two atoms. If the electrons present in the AO’s have parallel spins, no bond formation will occur, and no molecule will be formed. 3.
If the AO’s possess more than one unpaired electrons, more than one bond can be formed. Thus, in N2 molecule there are three bonds, since N atom has three unpaired electrons. N = 2s2, 2px1, 2py1, 2pz1 4.
Electrons already paired in the valence shell cannot take part in the bond formation. They can only take part in the bond formation if they can be unpaired with the use of lot energy. This is seen in the case of Phosphorus, which forms PF5 by sharing 5 electrons(three from the three unpaired P electrons and two from paired s) 5.
The strength of the covalent bond is related to the extent of overlapping of the atomic orbitals. The more the two bonding orbitals overlap, The more the bonding electrons are concentrated between the nuclei where they can minimize the nuclear repulsion and maximize the attractive forces between themselves and both nuclei jointly. Thus, greater the overlap of the atomic orbitals, the greater will be the strength of the resulting covalent bond. Types of Overlap
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Depending upon the type of atomic orbitals involved in bonding, different types of overlap can be seen. Thus, there may be σ or a π covalent bond. 1. Sigma (σ) bond
A covalent bond which is formed between two atoms by the overlap of atomic orbitals along their axis (end to end or head to head) is called as σ bond. All sigma bonds have axial symmetry. Sigma bond is formed by s-s, s-pz and pz – pz overlap. 2. Pi (π) bond
A covalent bond which is formed between two atoms by the...
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