Question

the molecules cis-2-pentene and trans-2-pentene are an example of a pair of ______.
○ isotopes
○ stereoisomers
○ constitutional isomers
○ identical molecules
○ unrelated molecules

Explanation:

1. Analyze "isotopes": Isotopes are variants of atoms with the same number of protons but different neutrons. cis - 2 - pentene and trans - 2 - pentene are molecules, not atoms, so this is incorrect.
2. Analyze "stereoisomers": Stereoisomers are isomers with the same molecular formula and connectivity but different spatial arrangements. cis - 2 - pentene and trans - 2 - pentene have the same molecular formula ($C_5H_{10}$) and connectivity (double bond at the 2nd carbon in pentene), but differ in the spatial arrangement of groups around the double bond (cis has groups on the same side, trans on opposite sides), so they are stereoisomers.
3. Analyze "constitutional isomers": Constitutional isomers have the same molecular formula but different connectivity. cis and trans - 2 - pentene have the same connectivity, so this is incorrect.
4. Analyze "identical molecules": cis and trans isomers have different spatial arrangements, so they are not identical.
5. Analyze "unrelated molecules": They have the same molecular formula and are related as isomers, so this is incorrect.

Answer:

B. stereoisomers