Grade 11 - Chemistry - Chemical Bonding Moecular Structure - Covalent Bonding

This type of valency involves sharing of electrons between the combining atoms to achieve stability through duplet or octet configuration.The electron pair forming the bond, being contributed equally by the combining atoms known as covalent bond.
Example:

Covalency:

a) The number of valency electrons which an atom can share with electrons of other atom/atoms to form covalent bond(s) is called its covalency.
b) It is also the number of covalent bonds formed by the atom of the element with other atom/atoms.

Lewis Dot Structure and Covalent Bond:

i) All atoms in a formula will have a total of eight electrons by sharing in the valency shell except hydrogen atom which forms the largest number of bonds with other atoms placed in the center of skeleton structure.
ii) Presentation of structure with the help of valency electrons as dots are known as Lewis dot structure.
iii) In such presentation the number of valency electrons as well as number and kinds of bonds are shown but not the spatial geometry of the molecules.
iv) Except hydrogen and hydrogen like species, Lewis dot structure shows that the central element achieve noble gas configuration(octet configuration).

Lewis Dot structures of few compounds

Molecular Formula Lewis dot StructureFailure of Octet rule:
There are several molecules having covalent bonding in which octet rule is not obeyed. In some compounds it is less than octet and in some more than octet.
Less than Octet:
i) Becl2 molecule in which beryllium has four electrons.

Valency Bond Theory

The theory was put forward by Heitler and London.
i) Atomic Orbitals of two atoms of same or different atom(s) overlap with each other forming a bond.
ii) Orbitals must have energy close to each other.
iii) Must contain unpaired electron in the overlapping atomic orbital.
iv) Electrons in combining atomic orbitals must be of opposite spins.
v) Probability of finding the electron pair is maximum in the new overlapped orbital.
vi) Covalent bond energyarises due to electrstatic attraction between nuclei and the electron densityin overlapped atomic orbital. Also some energy is released due to piring of electrons of opposite spins.
vii) More the overlapping (high overlapping integral value) greater the bond strength due to shorter bondlength,
Pauling and Slater rule:
i) The extent of overlapping depends upon nature of orbitals and as well as nature of overlapping.
ii) Valency shells closer to the nucleus , more will be overlapping.
iii) Sub shells of same energy level and more directionally concentrated shows greater overlapping, hence bond energy 2p-2p> 2s-2p> 2s-2s Energy Concept:
i) Atoms interact with eachother in the process of combining with each other to minimize their energy.
ii) The formation of hydrogen molecule from atoms, they interact with each other with release of energy while they are at optimum distance.
iii) The optimum distance is the bond distance, if they come more closer there is repulsion due to interaction with nucleus of both atoms and as well as due to electron-electron repulsion.
iv) The decrease in potential energy takes place during formation of hydrogen molecule.
v) In this case the energy of formation is equal to the energy of dissociation, which is very high in the case of hydrogen, i.e. 433 kj /mole.

Overlapping

i) In V.B. theory the overlapping of atomic orbitals of the combining atoms is a must for bond formation.
ii) The bond strength is attributed to the extent of overlapping and as well as size and shape of the atomic orbitals.
iii) The overlapping orbitals must have electrons with opposite spins.
iv) The overlapping orbitals result in decrease of energy.
Types of Overlapping and nature of Covalent bonds:
Covalent bonds are formed due to two types of overlapping, i.e. face to face overlapping and lateral overlapping leading to the formation of sigma- and pi- bonds.

i) Face to face overlapping of atomic orbitals leading to formation of sigma bond.
ii) In this case extent of overlapping is greater, leading to formation of more stable and strong bond.
iii) Hybridisation depends on sigma bonds
iv) Al single bonds between the atoms are sigma bonds.
v) Sigma bond is formed by overlapping of s-s , p-p and s-p atomic orbitals.
vi) Overlapping of hybrid orbitals also form sigma bond.
vii) Shape and geometry of the molecule formed depend on sigma bonds.

Pi- Bond:

i) Lateral overlapping of atomic orbitals leads to formation of pi-bond.
ii) Pi-bond is formed along with sigma bond, in which the electron cloud lies above and below the plane of sigma bond.
iii) As overlapping takes place on both sides of the internuclear axis, free rotation of atoms around pi-bond is not possible.
iv) In this case overlapping is scanty and thereby bond is less stable.
v) The reactivity of the molecule increases in which pi-bond is present as it is less stable.
vi) Hybridisation of central elements dose not depend on pi-bond.
vii) Pi-bond does not effect the shape of the molecule in which it occurs.