How many structural isomers are possible for heptane, octane, nonane and decane?

Hint: Alkanes are saturated hydrocarbons consisting of only carbon and hydrogen atoms. Alkanes exhibit only chain isomerism which comes under structural isomerism. Up to seven numbered alkanes, the number of structural isomers is $ {2^{n - 4}} + 1 $ where n is the number of carbon atoms.

Complete answer:
Alkanes consist of only single bonded carbon and hydrogen atoms. There are no functional groups or double bonds. Hence it does not show any positional or functional isomerism.
Heptane is an alkane with seven carbon atoms, the simple formula for the number of structural isomers is $ {2^{n - 4}} + 1 $ where n is the number of carbon atoms. By substituting the value of n, we will get $ {2^{7 - 4}} + 1 = {2^3} + 1 = 8 + 1 = 9 $
Thus, the number of structural isomers is possible for heptane is $ 9 $
Octane is an alkane with eight carbon atoms, the simple formula for the number of structural isomers is $ {2^{n - 4}} + 2 $ where n is the number of carbon atoms. By substituting the value of n, we will get
 $ {2^{8 - 4}} + 2 = {2^4} + 2 = 16 + 2 = 18 $
Nonane is an alkane with nine carbon atoms, the simple formula for the number of structural isomers is $ {2^{n - 4}} + 3 $ where n is the number of carbon atoms. By substituting the value of n, we will get
 $ {2^{9 - 4}} + 3 = {2^5} + 3 = 32 + 3 = 35 $
Decane is an alkane with nine carbon atoms, the simple formula for the number of structural isomers is $ {2^{n - 4}} + n + 1 $ where n is the number of carbon atoms. By substituting the value of n, we will get
 $ {2^{10 - 4}} + 10 + 1 = {2^6} + 1 + 10 = 64 + 1 + 10 = 75 $

Note:
The structural isomerism means the compound or molecule with the same structure but difference in the chain or position or functional group. Those structures are known as structural isomers and the phenomenon is known as structural isomerism. As alkanes exhibit only chain isomerism the chain isomers are the structural isomers.