state the number of sigma and pi bond formed. explain bond enthalpy and bond length
Answers
Answer:
Sigma () Bond
This type of covalent bond is formed by the end to end (head-on) overlap of bonding orbitals along the internuclear axis. This is called as head-on overlap or axial overlap. This can be formed by anyone of the following types of combinations of atomic orbitals.
s-s Overlapping
In this case, there is an overlap of two half-filled s-orbitals along the internuclear axis
s-p Overlapping
In this case, there is an overlap of half-filled s-orbital of one atom and half-filled p orbital of another atom along the internuclear axis.
This type of overlapping can be seen in the formation of Methane CH4, Ammonia NH3, water H2O
p-p Overlapping
This type overlapping takes place between one half-filled p orbital with another half-filled p orbital along the internuclear axis
Pi (π) Bond
During the formation of Pi bonds, atomic orbitals overlap in such a way that their axes remain parallel to each other and perpendicular to the internuclear axis
During π bond formation atomic orbitals undergo sideways overlapping which gives saucer type charged cloud above and below the internuclear axis.
Note
- All single bonds are - bonds
- Multiple bonds contain one -bond and the rest are π-bonds.
- π-bond is never formed alone. First, a -bond is formed and then the formation of the π- bond takes place. (Exception: C molecule contains both π-bonds)
- A sigma bond is always stronger than pi-bond because the extent of overlapping of atomic orbitals along the internuclear axis is greater than sideways overlapping.
- Electron cloud of -bond is symmetrical about the internuclear axis while that of π-bond is not.
- Free relation about a -bond is possible but that about π-bond is not.
- The less pi-bonds the more stable the compound is.
- The more the number of Pi-bonds the compound is more reactive
Bond Length
The average distance between the centres of nuclei of bonded atoms is called bond length. It is expressed in terms of picometer (1 pm = 10-12m) or Angstrom (1 Å = 10-10m).
In the covalent compound bond length is the sum of their covalent radii.
Example: Consider an HCl compound, the bond length is d= rH + rCl
In the ionic compound bond length is sum of their ionic radii (d = r+ + r–)
Factors affecting bond length.
(i) Size of the Atoms.
The bond length increases with increase in the size of the atoms. For example, bond lengths of H-X are in the order
HI> HBr > HCI> HF
(ii) Multiplicity of Bond.
The bond length decreases with the multiplicity of the bond.
(iii) Type of Hybridization
As an s-orbital is smaller in size, greater the s-character shorter is the hybrid orbital and hence shorter is the bond length.
Bond Energy or Bond Enthalpy
The amount of energy required to break one mole of bonds of a particular type so as to separate them into gaseous atoms is called bond dissociation energy or simply bond energy.
Bond energy is usually expressed in kJmol-1
Further, the greater the bond dissociation energy stronger is the bond
Factors affecting bond energy.
(i) Size of the Atoms
Greater the size of the atoms, greater is the bond length and less is the bond dissociation energy, ie., less is the bond strength.
(ii) Multiplicity of Bonds
For the bond between the same two atoms, greater is the multiplicity of the bond, the greater will be the bond dissociation energy. This is because atoms come closer and secondly, the number of bonds to be broken is more.
(iii) Number of Lone Pair of Electrons Present
Greater the number of lone pairs of electrons present on the bonded atoms greater is the repulsion between the atoms and hence less is the bond dissociation energy.