# Why does the cyclization of open-chain glucose occur via the C5 hydroxyl group?

In converting glucose from its acyclic (open-chain) form to either cyclic form (alpha or beta) why is the C5 hydroxyl group used instead of the bottom-most hydroxyl group?

Also, are these structures considered isomers? All sources I've seen only say "form" or "representation."

Including the linear chain form they are all isomers (wikipedia), as they have the same chemical formula. More specifically the relationship between $\ce{linear <-> pyranose <-> furanose}$ is called constitutional or structural isomer.
The $\ce{\alpha-D <-> \beta-D}$ forms are diastereomers, or more specific diastereomeric conformers.