Chemistry: Chemical Bonding: Bonding in Ethyne and Examples of Resonance (For CBSE, ICSE, IAS, NET, NRA 2022)

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Bonding in Ethyne (Acetylene)

  • In acetylene, sp-hybridization bonding is present.
  • One 2s and one 2p orbitals hybridize to give two sp-hybridized orbitals.
  • This leaves two mutually perpendicular unhybridized p orbitals each on both the carbon atoms.
  • The carbon – carbon bond is formed by sp - sp overlapping with each other.
  • The remaining sp orbital on each carbon overlaps with the 1s orbital of hydrogen to give C-H bonds.
  • The unhybridized p orbitals each on both the carbon atoms overlap sideways to give two Π-bonds.
Bonding in Ethyne

Resonance

  • The Lewis structure of a compound can be written more than one structure with the electronic requirements.
  • For example, we can write two Lewis structures for ozone molecules, O3 such as:
Resonance Oxygen Bond
  • Both of these structures satisfy the octet rule and have a double bond on one side and single bond on the other side of the central oxygen.
  • According to these structures one oxygen-oxygen bond (O = O) would be shorter than the other (O – O) .
  • Experimentally both the oxygen-oxygen bonds are found to have the same bond length (128 pm)
  • These experimental observations can be explained with the help of the concept of resonance.
  • The alternate Lewis structures are called canonical structures.
  • These are separated by a double headed arrow () .
  • All canonical structures must have similar nuclei position, number of bonding and non-bonding electrons and similar energy. The actual structure is the resonance hybrid of all the contributing canonical or resonating structures.
  • The resultant structure is stable.
  • The bond lengths and energies have values which are intermediate between single and double bonds.
  • Sometimes the structure of a resonance hybrid is shown with help of representation of bonds by broken lines or dashes.
  • The structure of resonance hybrid of O3 is shown below:
Resonance Molecules
Resonance Molecules
  • In molecules or ions containing more than two atoms and having double bonds or triple bonds, there are pi (Π) bonds.
  • The electrons involved in Π-bonds are not localized between two atoms but are spread over the entire structure (or the portion of the structure) which has only sigma- (σ) bonds.
  • This spreading of Π-electrons is known as the delocalization of the electrons.
  • Broken lines or dashes depict the region of delocalization of the Π-electrons.
  • Delocalization results in stabilizing the structure. So, the resonance hybrid is more stable than the contributing canonical structures.

Examples of Resonance

Carbonate Ion (Co32-)

The 3 possible canonical structures of carbonate ion are:

Carbonate Ion

Sulphur Dioxide (So2)

The 2 canonical structures of SO2 are shown below:

Sulphur Dioxide

Dinitrogen Oxide (N2O)

The three canonical structure of dinitrogen oxide are shown below:

Dinitrogen Oxide

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