Misc: Penicillin is derived from what?

Questions

What name is given to the central single wedge-shaped block at the top of an arch, which is essentially a central voussoir?

Who, in Greek mythology, was condemned to eternally roll a stone uphill, where it continually rolled back down again?

Which chemical element is named after the Greek word for 'green'?

According to the Gospel of Saint John, what 'lay on the other side of the Brook of Cedron'?

On which river is the city of Florence situated.?