Question

Amongst the following molecules, the one with the highest bond multiplicity is: -
(A) Ethene
(B) Ethyne
(C) Ethane

Answer 1

Hint : The suffixes -ane, -ene, -yne are all used for IUPAC Nomenclature of the hydrocarbon having single, double and triple bonds respectively. The single bonds present in this compound are the sigma bonds while the double bonds are both the sigma and pi bonds.
We can say that the multiplicity of a chemical bond is the number of electron pairs that occupy the region between the two bonded atoms in bonding molecular orbitals.

Complete Step By Step Answer:
The bond multiplicity of any chemical bond is defined as the number of all the paired electrons present between the two atoms that are bonded together. To calculate the bond multiplicity we need to find out how many electrons make up a given bond.
In the question, we are asked about the bond multiplicity of three hydrocarbons named ethane, ethene and ethyne.
According to IUPAC Nomenclature the hydrocarbon two carbons uses the prefix eth, and therefore we can say that all of the given hydrocarbons have 2 carbon atoms. We also know according to the nomenclature that the suffixes -ane is given when there is only a single bond, -ene when there is a double bond between the carbon atoms and -yne when there is a triple bond between the atoms.
Since ethane contains only a single bond, it means that they just have one pair of electrons. Ethene is a double bond, therefore, they will have two pairs of electrons and ethyne, having a triple bond will possess three pairs of electrons.
Bond multiplicity is the number of electrons pair between, in the above case, carbon atoms. Thus we can say that Ethyne has the highest bond multiplicity.
So the correct option is (B).

Note :
Do not confuse bond multiplicity with bond order. The difference between bond multiplicity and bond order is that bond multiplicity refers to the number of possible orientations of the spin of energy level in an atom, whereas bond order refers to a measurement of the number of electrons in the molecular model of a chemical bond.
The concept of multiplicity is very important in quantum chemistry, whereas the concept of bond order is important in molecular dynamics.
The equation for the determination of multiplicity is $2s + 1$ where s is the total spin angular momentum. The equation for determining bond order is
$\Rightarrow Bond{\text{ }}order{\text{ }} = {\text{ }}\dfrac{{{\text{(Number of bonding electrons - Number of antibonding electrons) }}}}{2}$