Answer:
A structural isomer is a molecule that contains the same quantity of elements (the same molecular formula) as another molecule, but they are arranged in a different way. These arrangements typically involve the branching of one or more substituents from the main chain. Isomers have different physical and chemical properties.
Octane is hydrocarbon (structure that contains only carbon and hydrogen) with a total of 8 carbons (note the prefix 'oct-'). The carbons are all attached via single bonds (not the '-ane' suffix). The structural formula for octane is:
CH3 CH2 CH2 CH2 CH2 CH2 CH2 CH3
To draw an isomer of octane, remove one of the carbons from the main chain and place it on one of the remaining carbons inside the chain (i.e. not on the first or last carbon of the chain). This will create a branched hydrocarbon chain that has the same number of carbons and hydrogens as octane (C8 H18). Placing the carbon on carbon 2 of the new chain results in:
CH3 CH CH2 CH2 CH2 CH2 CH3
(hope it helps)