Bonding in molecules forms the basis of elucidating the structure of molecules , which in turn dictates the properties of substances.In this post we shall learn about second kind of primary bonds – The Covalent/Molecular Bonds.
P2. THE COVALENT BOND.
This bond is due to sharing of electron pairs between atoms.The atoms involved have same/similar electronegativity.
Watch this video to understand how the sharing takes place-
e.g. – Methane molecule is formed by sharing of electrons between one carbon and four hydrogen atoms. The sharing ensures that carbon atom’s octet and hydrogen atom’s duplets are complete. This renders stability to both atoms and a new molecule is formed .
There are mainly three theories explaining the formation of covalent bonds – Valence bond theory (VBT), hybridization theory and molecular orbital theory.The first two theories help explain the shape and overlapping of orbitals and the later theory describes the molecular orbitals in detail .These three theories together, give us a clear picture of bonding in covalent bonds. These theories are discussed below –
A) VALENCE BOND THEORY(VBT).
This theory was first postulated by Walter Heitler(studied under Arnold Sommerfield, worked with Neils Bohr in Copenhagen and Erwin Schrodinger in Zurich) and Fritz Wolfgang London(‘London dispersion forces‘ fame, 5 times nominated for Nobel prize in Chemistry) in 1927.
Postulates of VBT –
- Condition for bond formation – A covalent bond is formed when adjacent atomic orbitals of two atoms containing an unpaired electron, with opposite spins, overlap.
- Binding force of the bond – Chemical bonds form when two atoms approach each other and the attractive forces are greater than the repulsive forces.
After the overlap, the electron density increases in the region of the overlap i.e in the region between the nuclei of two atoms.Due to this overlap , the repulsion (like charges repel each other) between two nuclei is reduced and the attraction between the nuclei and the valence electrons increases.
- Stability of the system – Whenever repulsive forces decrease and attractive forces increase, there is decrease in the energy of the system.This is because the attractive forces bind the molecule together and render stability to the system (more stability = decrease energy). The overlapping takes place only at equilibrium internuclear distance , where the repulsion is minimum. If the atoms come more closer than the equilibrium distance, the repulsive forces increase making the bond less stable.
4.Energy of the overlapping orbitals – The energies of the overlapping orbitals should be nearly the same.
5.Strength of the bond – Greater the overlap , stronger is the bond.
6.Number of covalent bonds formed –
# of bonds formed = # of unpaired electrons .
e.g.- Hydrogen has 1 unpaired electron so it forms one bond ,oxygen has two unpaired electrons so it can form two bonds.Nitrogen has 3 unpaired electrons so it forms 3 bonds.
7.Directional nature of the bond – We have already learned that s- orbitals are non-directional (as they are spherical). However, p- , d- , f- orbitals have a particular shape and orientation in space.Thus, the geometry of the molecules depends on the orientation of the overlapping orbitals.The orbitals are oriented in a way where they are as far away as possible from each other, ensuring minimum repulsion and have maximum symmetry.
The valence bond theory helps reveal how overlapping takes place in molecules. Let us discuss in the next post how exactly this happens, with the help of examples.Till then,
Be a perpetual student of life and keep learning …
Good day !
References and further reading –
Image source –
- By GFHund – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=11197534