Identifying pentane isomers Question
Which is not considered an isomer for pentane?
A) $\ce{CH3CH(CH3)CH2CH3}$
B) $\ce{CH3CH2CH(CH3)CH3}$
C) $\ce{CH3CH2CH2CH2CH3}$
D) $\ce{CH3CH=CHCH2CH3}$

I knew A and B were obviously isomers. I knew D was not an isomer, because the definition of isomers in my book states that isomers &quot;are two DIFFERENT compounds with the SAME molecular formula&quot; and that structural isomers are compounds with SAME molecular formula but DIFFERENT structure.
I was very confused with the choice C since it was IDENTICAL and did not match the definition. What confused me more is that there were practice questions in the book that gave like 4 examples of compounds and asked if  they are isomers or identical compounds.
So, are identical compounds considered isomers or not?
Identical compounds are not considered to be isomers. However, the following explanation should clear up any confusion from your question.

A and B are both isopentane and C is n-pentane (normal pentane). These make up two of the three isomers of pentane, the third being neopentane ($\ce{CH3CH(CH3)2CH3}$). The question is referring to pentane as $\ce{C5H12}$, a group of isomers in which n-pentane ($\ce{CH3CH2CH2CH2CH3}$) is included and not identical to.