What is a valence bond? Chemistry Notes for Class 11
VALENCE BOND – The attraction between atoms, ions or molecules that results in the formation of a chemical compound, is termed as a chemical bond. A strong chemical bond is formed from the transferor sharing of electrons between atoms. The electrostatic attraction between the protons in nuclei and the electrons in the orbitals determines the strength of a chemical bond. The strength of chemical bonds varies, as “strong bonds” or “primary bonds” such strong bonds are seen during formation of covalent, ionic and metallic bonds, and “weak bonds” or “secondary bonds” are present in dipole-dipole interactions, the London dispersion force, and hydrogen bonding.
Chemical bonds are formed when two atoms approach each other and the attractive forces are greater than the repulsive forces.
The chemical bonding is influenced by the attractive forces, repulsive forces, interatomic distance. It is also dependent on the shapes of orbitals in atoms and molecules, the number of valence electrons available in the bonding atoms, and the relative energies of the orbitals involved in bond formation. The two models of chemical bonding are valence bond theory and molecular orbital theory.
Read another topic for class 11 Atomic Theory
VALENCE BOND THEORY
A covalent bond is formed between the two atoms by the overlap of half-filled valence atomic orbitals from each atom.
Heitler and London in 1927, put forward the valence bond theory. This is mainly based on the concepts of atomic orbitals, electronic configuration of elements, the overlapping of atomic orbitals, hybridization of atomic orbitals. The overlapping of atomic orbitals leads to the formation of a chemical bond. The electrons are localized in the bond region which is due to overlapping.
The valence bond theory depicts the electronic structure of molecules. The theory suggests that electrons fill the atomic orbitals of an atom within a molecule. It also states that the nucleus of one atom is attracted to the electrons of another atom.
The basic doctrines of the valence bond (VB) theory are:
- The covalent bond is formed when overlapping of two half-filled valence orbitals from two different atoms. The overlapping results in the increase in electron density between two bonded atoms, thus giving stability to the molecule.
- If the atomic orbitals have more than one unpaired electron, more than one bond can be formed and electrons paired in the valence shell cannot take part in such a bond formation.
- A covalent bond is directional i.e. atoms bonded covalently attains specific orientations in space relative to one another. As a result, molecules in which atoms are bonded covalently have certain fixed shapes.
- There are two types of covalent bonds: a sigma bond and a pi bond. The covalent bond formed by sidewise overlapping of atomic orbitals is known as pi bond whereas the bond formed by overlapping of atomic orbital along the inter nucleus axis is known as a sigma bond.
Limitations of Valence Bond Theory
- This theory was unable to explain the formation of a coordinate bond, where the shared pair of electrons is contributed by only one of the combining atoms.
- It could not explain the paramagnetic nature of oxygen molecules.
- The directional nature of covalent bonds could not be elucidated.
- The formation of structures of several compounds involving resonance and hybridization was not clear.
For more information, Class 11 video lectures, study material, sample papers, Chemistry notes for Class 11, register with Takshila Learning.