Answer to Question 1
Spiral tracers are young objects, formed recently, astronomically speaking. O stars, for example, live for only a few million years. Their typical orbital velocity is about 200 km/s, so they cannot move more than about 500 pc in their lifetimes. This is less than the width of a spiral arm. Because they don't live long enough to move away from the spiral arms, they must have formed there.
Answer to Question 2
hydrogen; helium