Pentane has how many carbon atoms in its chain?

• A. 3
• B. 4
• C. 5
• D. 6

Answer: (C)

In alkane names, the prefix tells you how many carbon atoms are in the longest continuous chain. The prefix pent- means five, so pentane has a chain of five carbon atoms. Picture five carbons linked in a row (two ends are CH3 groups and the three in the middle are CH2 groups), giving the saturated formula C5H12. This is longer than propane (three) and butane (four), but shorter than hexane (six), so the five-carbon chain is what defines pentane.